This study was aimed to assess how carcass size affects protein degradation in postmortem muscle and subsequent impacts on meat quality. Beef carcasses were randomly selected at slaughter and classified as heavy (> 430.9 kg; n = 22), average (351.5 to 385.6 kg; n = 23), or light (< 317.5 kg; n = 20). Troponin-T (TnT) and heat shock protein 70 (HSP70) were analyzed for protein abundance on 3-day aged longissimus (LD) and semimembranosus (SM) muscles and 14-day aged LD muscle. TnT degradation in heavy carcasses was greater in the d-3 SM muscle and less in the d-14 LD muscle. Carcass size was positively correlated with the 30 kDa TnT band in d-3 SM. Therefore, carcass size influenced the TnT degradation but had no influence on HSP70 protein activity in either LD or SM.