So I used to have a slightly large frame, and I got a professional fit for that. On that frame I had a 80mm stem and wanted to go for a longer stem on my new bike to improve handling. I use a 40cm handlebar, so that was quite twitchy. I got used to that fit and liked it a lot, but I really wanted to avoid going to big with my next bike.

Also I was riding a used 49cm bianchi pista (90cm stem) in the past 3 month and while for the first time it seemed to be small, with an extremely big drop (headtube is 10cm with headset cups), I easily got used to it. I tried it on bit longer rides (30 miles) and still felt ok.
Maybe that's also part of the problem that I was riding the bianchi recently and changed the way I feel on a classic road bike.

The caad10 48cm has 5mm shorter TT than the bianchi, but a less dramatic drop, even without spacers. The reach difference between the 48 and 50 caad10 is 10mm.

Why would it be better to ride a 50cm caad10 with a 90mm stem and a short reach handlebar than to ride the 48cm with a 105mm stem? Longer wheelbase? Better handling? Better weight distribution?

I tried a 52cm CAAD10 earlier and it definitely felt too big. Not super big, but big.