If you're comparing 2 or 3+ photos, why would you want or need a giant sidebar?
Yikes, it just gets worse.
i can tell you why: because this program is optimized for touchscreen.
and why is it optimized for touchscreen?
because apple THINKS their iOs way is the best way. and they forget, that an imac or mac book is NOT a touchscreen device!
this is actually quite scary to see, because obviously apple can't work out that they can not just compile an ios program to normal mac os x.
there is more evidence to this theory:
they force you to double click on a photo to go back. this is typical ipad usage, as an ipad has no esc button and you also only show pictures on the ipad. ESC is now "leaving the fullscreen mode".
what really surprises me also is that fullscreen modes were always considered as bad on the windows. nearly every program on the pc can be run in fullscreen but apple always refused to implement a fullscreen button. the green button behaves like it likes, some programs resize to some strange size, others to fullscreen. it was VERY confusing sometimes, but HEY- THIS IS APPLE! it can't be bad and it MUST be better than windows that way!
you just eat it, and don't think that it is bad. it is apple, so you "eat" it - no matter how unusable it is.