I noticed this problem as well. If I have D2R running for a long period of time, the memory continually expands. The other day I noticed the browser in my second monitor was extremely laggy and couldn’t load pages. It turns out that the reason why was because Diablo 2 was using 10,000 MB of memory. I quit and restarted and it fixed the problem, but this memory bloat happens basically every time I play, increasing slowly but surely.
I have to wonder if this also has something to do with why it often takes forever for the process to terminate after quitting the game, as many people have reported. We might be waiting for all this memory to be released.