HP45 fit smaller than HP35, but larger than UCLP. The HP45X and UCLX are the same size as their non-X counterparts, but have additional hip and thigh padding. As a defenseman, I prefer the X-models. HP45X is my favorite pant right now (even though I'm currently using a 9K girdle and shell).
I am 5'10" and 155 lbs with a 33' waist, and wear a medium HP45X. In UCLX, I can fit a large (it's a little loose but comfortable), and a medium. The medium is much more snug.
Compared to HP35 and HP70, the HP45 and UCLP are very light. I weighed both X-models and they came pretty close to the same weight. The spine protector on both models sometimes will snag on the back of my Tacks 7092 shoulder pads, but I jersey tuck behind the spine protector which alleviates that problem.
For anyone worried that the HP45 or UCLP don't protect well because they are light, I've blocked shots wearing my HP45 and didn't feel a thing.