I notice every Adobe Flash update is around 20 MB in size. Are all these updates cumulative? Or does each update simply replace the previous one?
The short, plain answer is that the "latest version" of FP is always the "current" installation package as of the time it's released, and replaces an existing earlier version if it exists on the target system. Whether you're "updating" FP or installing it for the first time, you use same installer package. If an earlier version is already installed on the computer, the installer removes that when it installs the latest version.
The longer, less straightforward answer is: unless I'm very confused (by no means impossible<g>), I think what you really meant was: "Are all these updates
incremental? Or does each update replace the previous one." As you wrote the questions, they both mean more or less the same thing.
Cumulative updating means rolling all the previous updates into one "installation", while incremental updating implies making only the most recent of a series of modifications. And in the strictest sense, fully
replacing an existing installation isn't really an "update" in the usual sense (like Windows updates, for example). Technically, one might say that the Flash Player software itself "has been updated", but when you "update Flash Player on your computer", the previous version is completely removed and fully replaced with latest version; the "updates" don't merely modify the existing installed program.