I've got three SCX10 IIs and I have different size 1.9" tyres to them, from 4.75" to 4.19".
The bodies are different sizes, and I made a "comp" rig from one by removing the rock sliders and bumpers so it's sort of poor mans Capra.
Tyre size doesn't seem to matter that much although there's a noticeable difference, biggest performance difference is with a body that needs to be light to keep the center of gravity low.
Next is the size of the body that needs to be small so it doesn't get stuck on rocks, I didn't want to go full derp so I added a small bumper, leds, and a spare tyre.
With small RC4WD Wrangler MT/R 4.19" tyres, plastic beadlocks, and 7.2V 1200 NiMH it weights 2kg/4.5lbs.
It's ridiculous how tricky places it can drive compared to the bigger and heavier SCX10 IIs.
I tried bigger RC4WD Wrangler MT/R 4.75" tyres on metal beadlocks on it, but it raised the center of gravity and made it roll over easier.
Because of the metal beadlocks and bigger tyres the weight of the tyres more than doubled but it didn't help with climbing or sidehilling.
Here's video I made of the white SCX10 II "Sport", metal beadlocks with 4.35" Pit Bull Berserkers and huge 8.4V 3000mAh NiMH.