Andre the Giant was a genuine "giant" with a massive frame size and massive hands. Daniel Cornier is a stocky, but not very large man. Very different.
Sheer weight, no matter the source, is at the very least an anchor. Having a heavy and sturdy anchor matters both in wrestling and in lifting anything. But actual bio mechanical strength comes from the frame, the attachments and lever points.
Point being: weight does matter; even if it's fat or water it still provides anchor. But frame and bio mechanical strength matters more if I had to chose between the two. Obviously, both are important for wrestling at the highest level; sumo wrestlers being an example of this.
Actually, on the topic of sumo wrestling: depending on the style, a complete fatass with a mediocre frame can be effecive. One of the current ozekis, Takakeisho, is as complete fatass with a mediocre frame, but is a pusher thruster and just has to be reasonalby explosive and push his oppenent out really fast. Daiesho is similiar. and they can be effective at this even against oppentents with much more imposing frames. With that said to be a muwashi fighter ie grab the muwashi (the "belt" the wrestlers wear), a wrestler has to have a big and imposing frame like Tochinoshin or newcomer Kinbozan.
To be honest, anybody interested in the importance of weight, or the weight vs frame size, or weight AND frame size topic, should watch some sumo wrestling. The March Tournament is almost over but not yet, can find the matches on youtube or the NHK network