CLs become one perforce commit, but their lifetime is more like a git branch. I try to keep CLs under 100 delta lines. (In contrast, I try to keep git commits around 10 delta lines.)
People that only write one CL a month have probably written a fairly large amount of code, but are being blocked on reviews. When I see a review for more than 100 lines of code, I immediately think, "I'll do this later" or "Oh good, I'm only CC'd on this review; ignore." If the person doesn't have anything else to work on, this blocks him until I feel like diving into 1000 lines of code I've never seen before. Conversely, if I got one 30 line CL every day for a month, I would probably immediately review each, turning the one-CL-a-month guy into a 30-CLs-a-month guy with the same amount of code.