Maybe I am missing something, but Mario 64's boss fights, for the most part were "Just around the corner" because the stages were so small. Heck I believe the only boss fights that you reappeared close by were the ones that took longer to get to, like Bowser and I believe the desert boss.
Regardless I think Galaxy is much harder than Mario 64, even with all the one ups (But ALL the 3D Mario games have had easy 1ups). Some of this challenge, sadly, comes from the camera which can get flat out confusing at times, I definitely prefer a controllable camera wherever possible, which is probably why I love the larger stages a bit more. Another area that is tons harder than Mario 64 is the boss battles, in Mario 64 they are virtually a cake walk now, here there are some truly challenging (yet fun) boss battles, something I enjoyed about Sunshine as well.