Tom Gaylord did a blog about pellets for the Diana 52 on his blog http://www.pyramydair.com/blog/2006/02/best-pellets-for-diana-rws-52-how-to.html

I'm of the notion that too light pellets do more harm than too heavy pellets. Heavy pellets may or may not weaken a spring over time, but too light pellets or too loose pellets might let the piston slam into the chamber and do far more than just weaken a spring.