Whatever bitcoin node Cashapp uses to verify coins that are sent to it is a "big node" in the sense that has a lot of opportunities to reject coins that don't follow rules Cashapp thinks are bitcoin rules.

We all rely on a node (ours or someone else's) to make sure that coins we receive are following Bitcoin's rules. If you only receive a little bit of bitcoin every once in a while, your node is "small" in that not very much economic activity will be affected if you demand coins that follow a specific fork. If Cashapp only accepts coins that follow a specific fork, a lot of economic activity is affected.

Replacing node with rules doesn't quite describe what is happening. If someone keeps their coins at a custodian, they don't really have much say over consensus changes, even if they do have a lot of economic activity. Similarly, if a lot of people rely on a specific node (say Mempool.space's node) or a certain public electrum server, the rules those people care about may not have much relevance (unless they are willing to spin up their own node to make sure coins they receive follow such rules). I think it does make more sense to talk about nodes rather than rules here.