Salmotrutta,
A small pond I fish in southern utah has produced browns to 20 pounds. It isn't more than 25 feet deep, and probably 7 acres in size. There are two forage species in there. Utah chubs, and redside shiners. The browns are self sustaining. I would think If you are catching small fish, the problem might be that there are too many of them, or they are being harvested before they make the switch to eating the shiners.
Also, are you using small lures or large lures? The adage of big baits for big fish holds true most of the time. Can you use live shiners? Have you taken notes of what the fish you are catching have been eating? They may be eating insects or amphibians or something other than the shiners, and only switch to shiners after reaching a certain size.
Big browns are very wary, and often feed only in very dim light or at night. Probably even more so than walleyes. Are you fishing dusk and dawn? If so, try twilight or nighttime.
Good luck.
Fishrmn