heres an example (using info from mikeswi from the link further up):
W= size of Laser MK V and E= size of a Hyperwarp III, C = size small hull.
Advanced mini(no shrinker): C = 36, E = 6, W = 5
Expert mini (No Shrinker): C = 40, E = 7, W = 6
so, say you had a ship, 6 W and 1 E, taking up total size 36 (completely full small hull) with advanced miniaturization.
you get expert miniaturization, and these same components take up 6*6+7=43 space, but hull capacity has only gone up to 40 - -your ship is now illegal, you must REMOVE a component after having researched more miniaturization!
This is just one concrete example, there will be many other combinations where this happens (and others where miniaturization would work as expected)
The problem (as others have stated) is with decimal rounding and the size mod which adds a percentage of hull size to component size. - when you research a level of miniaturization, certain compnents will jump up a size when placed in a certain hull, instead of a smooth increace. So combinations of components many of which have just 'jumped' after a given miniaturization tech can end up increasing by a larger percentage than the miniaturization percentage, and may no longer fit in a given hull they fitted in before.