How much credit should you get for beating smaller guys?

Size plays a role. It is the reason there are numerous weight divisions. When two fighters are equals in terms of speed and power, but one guy has the advantage in reach and height, he will likely win.

6'1 heavyweights arein short supply these days. Because when one fighter is 6'1 and weighs 210 in his peak, prime fighting condition, and another is 6'5 and weights 235, the bigger guy has the advantage because of height and reach.

There are exceptions, but generally, a smaller guy deserves more credit beating a bigger guy than vice versa.