My 6553 is 9 mm cased and in the classic 34 mm size. I find the proportions to be appealing. the case back is flat so the watch lays flat on my wrist. I had a 6595 with the older 95# movement, it's case had a rounded base and would roll a bit on the wrist. I prefer the 6553 , it's just more comfortable to wear. The 6553 is only 3 mm thicker than my 0071 which is a very thin watch.
I have an Air Command 40 mm case and don't feel it's to large, as a comparison the 6553 doesn't feel to small, the smaller watch is more comfortable under elastic cuffed jackets, fits perfectly under the cuff of a nice shirt. One point I find interesting is the crystal on both watches is the same width so both watches have the same watch face display size.
6553 picture of watch on my wrist;
My Trilogy;
Chet