Indeed, but there are other important factors besides size.

DSN have way bigger requirements in terms of signal to noise ratios, since they need to demodulate received signals.

Scott Tilley, on the other hand, seems to be mainly interested on detecting the presence of signals, not necessarily extracting information from it. Then, he could build a bigger dish, but he doesn't actually need one to achieve his goal.

I notice that another guy commented here about that. His comment is dead now, but he had a point. I found it informative indeed, I've always asked myself why spacecrafts "wasted" energy with carriers and never realized they make the signal easer to find.