I didn't say he was a shrimp. I just said he doesn't -- and never will -- have Brown's power or tackle breaking ability. Ronnie's power was rare, in my view. His problem was his hips and his vision, both of which were only average at best. But in his day he was one of more powerful running backs in the NFL and had good burst (though with limited top end speed).
Thomas is a slasher with elite feet for someone his size. But he's not a tremendously powerful back not only because of his pad level but because that's just the way he's built. In boxing some guys have powerful punches and some guys don't. Just as some guys have a really quick jab and others don't. It's not a height/weight/arm length thing, it's just the way they're built.