[oleg]

My impression is that once AI makes an advance in tech and decides to upgrade designs, it cycles throw hull sizes starting with smaller one.
For example, this is what I obsedved in my Last game (proportions mod): I am fighting Piundon, they have ship hull up to cruiser and shard cannon II. The moment they discover Shard cannons III, they create a new _frigate_ with SC III and label all light cruisers and cruisers obsolete. Next one or two turns, they upgrade design to destroyers, then LC and finally cruisers. I have been fighting battles in succesive turns against new frigates, then destroyers, etc.

When Piundon made next upgrade (after getting shield II), the cycle repeats itself. Apparently, if research is slow, AI will produce mostly biggest ships. I might be wrong, but it is what I saw in my games.