The size really doesn't matter in that respect, only the maximum discharge current might make a difference.
For example, I run my Amp using these two packs:
- 25C 3800mAh LiPo,
- 50C 5000mAh LiPo.
The first one can deliver 3.8*25=95A of current according to the sticker. The second one can deliver 5*50=250A of current. I feel only a slight difference between the two batteries. The maximum burst current my stock Amp can draw must be something around 100A or the difference in acceleration would be much larger. That, or my higher end battery's sticker is a lie
Are you sure your slipper clutch slips? Have you taken it fully apart? Maybe the pad is simply stuck to the clutch disk and no amount of loosening the spring will help. People have run the stock ruckus on LiPos with the stock power system without instantly eating up their plastic gears.