I fought a ranger outlander leader type and he was able to bearcharge me every round and lag me almost out despite being three sizes smaller then me. I was an enlarged giant, and he was an enlarged small. Is bearcharge supposed to be every round for a caverndweller explorer ranger? And is it supposed to be that effective if I'm, at the very worst case scenario, two sizes bigger then him, if not three?

Yes, that kind of ranger is supposed to be able to bearcharge that fast.

I can't imagine that's actually providing any lag though.

Caverndwellers get so called 'quick-bearcharge' which can go every round. But even with +1 size difference (with felar) I get 0 rounds of lag in 75% of cases. So that was either some awful RNG on your part OR, alternatively, you was lagged from something else (aka snares).