Is trade more lucrative in larger sized galaxies? Because in the medium ones I play often, Trade is essentially useless except perhaps very early to balance out early losses before income streams manifest (which I rarely do because the production required to make these ships could go to other improvements or research) or for relations purposes (which at the obscene level I play at, everyone hates me all the time anyway, and even if they do like me, they can still make war with me at the drop of a feathered pimp hat).
Longer answer: sort of. I currently make 650bc on trade and 2200 bc on tax
immense map 11 trade routes, all the longest I could make. (I play on high levels as well and the + to dip doesnt help me much either.)
Settings Masochistic Max CPU AI Rare EVERYTHINGSo about 1/4 of my income is from trade which seems fair to me BUT I only have 6 planets.

Only 4 of them are economy planets

so if I were to have the 50-300 planets that I get with higher planet settings, then the 600 BC of a 20K+ a turn economy would be nothing.
In short, relative to the total planets settings, trade routes make a difference only when there a generally few planets but much more so when on VERY large maps. Simply the distance involved makes ALL the difference. But that difference is a sneeze in a hurricane on abundant settings.