I've reread my post as it seems a bit long but I'm unsure if there's anything I can safely cut out.
Also, just for the record, Justin, calling Tough a low difficulty level isn't quite accurate. True, there are five above it, but then again there are six below it. In my book maso, obscene, and suicidal aren't so much difficulty levels as they are bonus levels, so I'd place tough at 3rd of 9, which isn't too bad.
At some point I gained the impression that we were talking about ship design as it is relevant to the higher difficulty levels. That may have just been myself and JustInsane (and maybe whismerhill) talking; I'm not sure. If that's not what we're talking about, then I apologize, as the holes in your argument are not that apparent during normal play.
You might want to consider the fact that in all of my examples thus far I have used cost equality as well.
As for your example of a 20 attack medium alongside your defense ship-assuming tech equality once more, for dynamic shielding vs. psionic beam we have our weapons essentially maxed out at particle beam II. Particle beam II takes up the same amount of space as psionic beam. You can fit a grand total of 7 psionic beams (8 size + 4% hull size for 8 + 1.92 or 9 total size truncated) on a medium hull at the above mentioned 40% miniaturization (67 space). Therefore, you can fit a grand total of 7 particle beam IIs onto your all-attack companion medium to your defense medium, resulting in 14 attack. You can try using harpoon (I) for roughly the same tech cost, but due to the size and the sizemod you're looking at 5 of them max in 67 space or 15 attack there.
So that's the first thing wrong with your theory.
One other thing you seem to have failed to consider is average damage. (As you have said a ship with 20 attack is sufficient to kill a medium. This is simply not true.) 84 attack doesn't mean it'll do 84 damage; on average it's a lot closer to 42. Statistically it's actually not quite 42, but that's close enough for our purposes, although it's worth noting that it becomes even more complicated when defenses are factored in. So your 14 attack ship above is going to average 7 damage in the round of combat it participates in before it is destroyed (unless your defense ship has a combat or warp bubble module on it, in which case, oops, your defense ship is going to be destroyed). A medium hull has 20 hp, not counting any bonuses. So let's say it takes you 3 hits to take out one of them. That's okay, your attack ships are cheap (325BC at 7 particle beam IIs and a medium hull) and you're suiciding them anyway, let's put three in the fleet instead of one, for a four on two battle! (Note: Two won't work-they'll both die first round if you're facing off against a 2-ship fleet.)
This way you'll get sufficient damage in the first round to take down a single medium psionic beam ship. So you lose 975BC to take out two ships that cost a minimum of 680BC each (4 psionic beams will, on average, one shot your attack ships) and perhaps as much as 1130BC each. The only catch is you need to invest another 3500 research in logistics to form this fleet. While it's worthwhile, it's time spent where your ships, if you have any, are getting their asses kicked. The same thing can be accomplished (you know, the losing three attack ships part) by using four attack ships. You lose two the first round while he loses one, with his second at 2/3 hp. You lose a third ship in the second round, but you destroy his ship in the process. This winds up saving you about 500BC per fleet, if you need multiple fleets, although you're still going to be paying out the ass for the logistics (see above). Did I mention particle beam II is a lot cheaper to research than particle beam II and dynamic shielding? Heyyyyyyyyy, maybe you can get your logistics higher going that route. Oh, you can? Sweet! And you still have tech points left over? Remind me why I want to go defense again?
Which brings us right back to the issue of using good defenses to combat evil weapons...which quite simply does not work.
Now I know I said the AI has difficulty researching ethical techs, but that doesn't mean it never will. And let me tell you, an AI fielding psionic beams is a scary sight.
To nitpick on a few of your above points: I assume all defense, one weapon ships because you need that much defense when facing off against evil weapons (or end of tree weapons, for that matter).
Additionally, if you're into large hulls already, while your opponents aren't (an extra 3500 research on top of everything else), then either you are the tech race leader, in which case you're good to go almost irrespective of what weapons/defenses you use, or, and this is more likely on anything beyond Tough, you're the single tech race loser, and you don't have enough shit to put in your ships to even justify using large hulls. 3500 less research in shields would place us at shields III, which is the same size as dynamic shielding but has 1/3 the defense. We don't quite have enough to get subspace rebounder, but I'll grant you that-which brings us to about 55% of the defense per space of dynamic shields on a medium hull-not quite 60% on a large hull.
If defense ships work for you, then feel free to continue to use them. However, if I might make a suggestion-try going evil next time, and incorporate normal defenses and evil weapons into your ship designs. See if it works out any better.
I'm going to end my thoughts now with saying, for the 31,000th time, that defenses need to be better.
While I'd love to argue with you more, I am presently largely unable to do so. My ISP has accidentally lost the DNS entries for a number of sites, and Stardock's entire web is included in that (in fact, that's the first part I noticed). It's possible it's just a down machine, but it's been several hours. I'm currently accessing the forums by way of TOR, as my primary proxy method (via Tunneling) is, you guessed it, inaccessible for the above reasons, but due to a number of things, this is insufficient for foruming. The main component is the bandwidth, as I'm pulling about 1-2KB/s here at the moment, if that, but it's also a major annoyance that (for those of you who don't know how TOR works), my IP as others see me continues to change, and it would appear that Stardock's network of sites uses IP-specific cookies. While in general this is not a bad thing, it means I have to log in somewhere in the vicinity of once per 3-6 minutes, which is about how long a page load is taking me at the moment. Let's just say it's not a good tradeoff.