Chaos has a much more difficult time replacing losses because they don't have a system of recruitment (homeworlds from which to select from, etc), and because they don't have the technology that the Empire does in the form of the Adeptus Mechanicus. Among other things, the AM tithes a small percent of gene-seed from all Marine Chapters and genetically clones it to eventually form entirely new chapters, manufactuers equipment, weapons, starships, and everything else. Chaos usually replaces Marine losses by haphazardly forcing captives to undergo initiation (forcing a much higher rate of failure among "candidates" ), or from defections of Loyalist chapters (which does still occur sometimes).