Tommy Lee may be aging, but he ain't shrinking -- in fact, his much younger gf wants you to know he's still living very large.

Brittany Furlan and the Motley Crue drummer are vacationing in Cabo San Lucas, and she shared a video of the gorgeous scenery. The most breathtaking view -- to Brittany -- is in Tommy's shorts.

We broke the story ... the couple presumably joined the Mile High Club in August, although they later claimed there was no sexy business.

This video makes that very hard to buy.