Re: Why different thickness on bars?
When you say 'thickness', I assume you mean width. as in a 1 1/4" vs 1 1/2".
on a lang hive, the frames are spaced apart from each other and the bees will draw out the comb as much as they need to, extending beyond the width of the top bar. It doesn't matter what the top bar width is because it is open above the frame for bee space. Also beeks will and can remove a frame from a hive if the space is getting too tight. Instead of 10 frames, only 9 and so on.
on a top bar, the top bar itself is the ceiling. There is no bee space above for bees to walk over. Therefore, observing that on brood comb, bees often draw out comb to about 1 1/4" thick and stores comb to about 1 1/2" thick, allows the top bars to be snugged together yet allow bee space fin between fully drawn combs for bees to move around the hive in.
Just the same, some folks only make their top bars 1 3/8" wide all the way splitting the difference between 1 1/4" and 1 1/2"
No, I am NOT a bee "Keeper". Anything I post is just my opinion. Take it easy and think for yourself.