Some people with large hands find putting the little finger under the grip helps.
Also, oversize grips will not be SASS legal though that may not bother you. An 1860 gripframe and grip is legal.
If you just want the grip to be fatter, some aftermarket grips are fatter than others. About 15 years ago I ordered some Altamont faux ivory grips for a 2nd gen Colt, and they were way too thick for my smaller hands, so I sold them.