Bike Fit: A Case of Evolving Preferences?

Last weekend we finally picked up my Francesco Moser fixed gear roadbike from Harris Cyclery, where it sat most of the winter after getting its bottom bracket replaced. I haven't been on this bicycle since December, riding the slightly-too-big-for-me Bianchi instead. And now that the Moser is back, guess what? It feels small! I was warned that this would happen once I got used to a larger frame, but it's still hard to believe. How can I feel cramped on a bike that felt "perfect" last year?

In addition to having a shorter top tube than the Bianchi, the Moser is also set up with a shorter stem. The difference between the total distance from the seat post to the handlebars on the two bikes is 3cm - which seems like a small number, but feels quite otherwise, at least to me. The handlebars on the Moser are also set quite a bit lower, which I think contributes to the cramped sensation. Combined with the lower bottom bracket, I almost feel like an adult on a miniature pony when I mount the smaller bike after the larger one. Would a longer stem alone solve this, or have I "outgrown" the frame itself? At the moment, both my Bianchi (with a 55cm top tube and 8mm stem) and my Rivendell (with a 57.5cm top tube and 6mm stem) feel just right, but how can I trust that "just right" feeling when it keeps changing?

It is said that preferences in bike fit "evolve" over time. But does there eventually come a point when they stabilise?  How many times in your cycling history has your idea of comfortable bike fit changed?