I assume that we are talking about an offline power supply here? Or is it a secret?
Anyway, the point is that C3 and C4 are carrying the full primary AC current, not just primary magnetizing, but also reflected secondary load current, which can be a lot.
So when I say "ripple current", I mean this full current.
Electrolytic capacitors have limits for maximum ripple current (which is mainly, but not only) derived from ESR. Yes, it's a power dissipation issue, but the ripple current limit also takes things into account like possible local hot spots in the cap.
I suggest you download some electrolytic cap datasheets and try to understand them fully.
Concerning half-bridge vs. full-bridge there is no clear-cut answer. Heatsinks also take up space, and drive for a full-bridge is somewhat more complicated. Things like form-factor come into play (if the thing has to be extremely flat, capacitors are perhaps not so cool...)