Almost everyone has Hi-DPI displays by now, and some Haiku design cues are from a time when DPI wasn’t that high. A laptop 720p screen felt humongous in the 2000s but nowadays it’s kinda cramped for current OSes.
I reckon the Trac theme follows the same criteria, and since devs use it daily they’re accustomed and don’t see any problem. But for new contributors, I agree the bugtracker looks clunky (intimidating, even) and might benefit from a UI revamp.
Slightly offtopic but related… This is something I do on EVERY Haiku installation otherwise I’d need a magnifier 
a) enlarge desktop icons size (usually 48x48)
b) enlarge UI fonts slightly (personally I set 14pt or more)