Naa, Shazier's injury had nothing to do with his size. He lowered his head into tackles. Bush and Spence both had bad knee injuries. That comes from twisting wrong, falling wrong, bad footing, so on and so forth. In all honesty, the bigger risk is not size but leg extension strength, at least according to this
study. Note, though, that study was with non-contact sports. But it does show a connection. Another
study looking at knee injuries across sports, and found most in skiing, football, basketball, and soccer. Note that only one is a contact sport. From this study, the injuries come more from loading the knee with torque while running, turning, so on and so forth. A couple of interesting gems from this article, learning better body position and focusing on strengthening hamstrings over quads are very important in cutting down such injuries
tl;dr, the issue really isn't smaller frames. It's muscle strength and bad body positioning.