What type of wheel is it? I have a robo and i tried a few wheels as the same happened with him.
I did try him with a silent runner when I first got him but he didn't seem to know what to do with it, so I swopped it for a pets at home one, but he kept flying off it, so then I tried him with one of these
Rodent Play Wheel, Free Standing, Large
and it was even worse, so gave him back the pets at home one... anyway fast forward a couple of months and I thought I will try him with the Silent Runner 9 inch again and this time he zoomed along and no flying out. He spends way more time on his wheel now too. I think he was just too little when I first got him, he weighs about 13-14g more than when I first got him (19g)
I think the theory is, the bigger the wheel for them the less chance of them flying off. So no, I don't think it is that the wheel is too small, it's maybe not quite big enough. Some robos can even run really well on 12 inch wheels. Just depends on the wheel and the robo.