Depends of hoe big they are and the defenition of too big differs from each person. I would say good ones are indeed hipnotizing. And for me the most perfect ones, are the ones of who fit exactly in my palm of my hand. Not bigger, not smaller.
As for the why they are hypnotizing, good question. Maybe some psychologist in here can answer, but then again, I doubt any sane person posts of MTF, and usually you have to be sane to be a psychologist, so I guess we have none...
it's way too easy, why would you need a psycho(logist) for that? Most've been fed by these, most would follow them up, type wise as their moms'.
You'd hardly see a guy, havng his mom's breasts tiny follow on to big boobed girls and the inverse, it's just simple psychology.
With the few exceptions that will come along to seal it ofc, especially here, but that's the general rule and it's pretty simple.