Previous studies demonstrated that the addition of biodegradable polymer microparticles to calcium phosphate (CaP) cement improves the cement's degradative behavior without affecting its handling characteristics, especially its injectability and moldability. We investigated the influence of molecular weight of polymeric microparticles included in CaP cement on implant degradation and bone formation in critical-sized defects. Forty rats received cranial defects filled with formulations of CaP cement and poly(DL-lactic-co-glycolic acid) (PLGA) microparticles. Microparticles consisted of 100% high- (HMW) or low-molecular-weight (LMW) PLGA or mixtures of these (25%, 50%, or 75%). Implantation time was 12 weeks. Porosity measurements showed that the 100% HMW group was significantly less porous than the other groups. Histology and histomorphometry revealed significantly greater implant degradation in the 100% LMW group. Defect bridging was mainly seen in the 75% and 100% LMW groups, with the highest amount of bone in the 100% LMW formulation. These results suggest that LMW PLGA microparticles are associated with better bone formation than HMW PLGA, which is most likely explained by the greater degradation of LMW PLGA microparticles. In conclusion, CaP cement composites with high percentages of LMW PLGA microparticles show good bone transductive behavior, with complete defect bridging. The 100% LMW group turned out to be the best formulation.