That's true, all other things being equal.
I'm a fairly sucky BJJ blue belt, but I would hang up my belt if I couldn't give any raw noob a run for their money in a pure grappling encounter no matter how big and strong they were. (Obviously everybody has off days, so if one of you is a 300lb powerlifter and hapens to visit me and squashes me into Australia, I'm having an off day. OK? )
The size and weight thing applies to all unarmed arts, but more so standing up IMO (whether grappling or striking). That's why newaza skills are so often advocated for smaller people - on the ground you minimise the number of varibles that come into play and are able to negate (to a certain extent) your opponent's strength/size advantage.
To Richard (Sapporo Ichiban) - 215lbs is light? You must be American! (I'm 5'10" and about 185lbs, I compete at 174lbs, and I'm one of the larger guys at my judo club)