The xp on the mobs is likely fine under "normal" circumstances. You get base xp solo. You get a bonus with a partner, then every person after that lowers the xp received. This is why trials are typically horrible xp from the mobs. If you go with with just one other person, the xp per mob is probably much higher, but of course, you may take too long to clear them out.
For this reason, they can't just raise the xp across the board as min/maxers would go into trials duo to get insane xp.
As mentioned above, trials are not a high xp/minute activity, probably by design. You're often there for other reasons such as achievements or gear collections.