Unused RAM is wasted RAM.

Gmail has an internal behavior that adapts its memory usage on available memory. It may fail in some specific situation (short and wide memory leak), but as a whole the idea is to use RAM as much as possible to make gmail faster.

And yes, gmail lacks of a "low memory consumption" button for those situation where automatic memory usage detection fails.

There is video of google explaining this voluntary behavior but I can't find it unfortunately.