I don't understand that remark. It isn't weight OR agility - you have modern athletes with both size/weight + ability. In a completely equipollent state: 6'2 220 guy vs 5-11 205 guy with 100% equal skill, you 10/10 grab the bigger guy.
The ideal is getting a bigger guy who covers more net with the same agility of a smaller netminder OR, the agility lost is barely significant to the gains of the size.
Bigger fills more net. Lots of defensive systems that keep shots low percentage, become even lower with a bigger guy. You can check out lists of goalie size now vs historical-- it's a big difference. Bigger guys are larger obstacles to 2nd attempts / rebounds and are physically stronger and harder to move out of net if refs are allowing some interference.
NHL is a game of inches. You want to be bigger when you go down in the butterfly. Smaller goaltenders will have space over the shoulders. Big guys shoulders will be flush with the crossbar (I know, I'm shifting to height now, my apologies), but also wider and fill out their equipment a little more, and the net hasn't gotten any bigger. Weekes did a presentation on this on the NHL channel. They drug a net out, the whole thing.