Size and Weight:
Along with Harpymimus, Pelecanimimus is what paleontologists call a "basal" ornithomimid, with less evolved features than the bigger, faster bird mimic dinosaurs of the late Cretaceous period like Ornithomimus. In this respect, the small size of Pelecanimimus makes sense, but what's most puzzling is this dinosaur's huge number of teeth--well over 200, the most of any theropod yet discovered, even including Tyrannosaurus Rex. (The only other ornithomimid known to have sported teeth is Harpymimus, and this dinosaur only had a mere dozen or so.)
We may never know for sure why Pelecanimimus had so many teeth, but we can speculate about why later ornithomimids were completely toothless. As these fast, ostrich-like dinosaurs learned to feed primarily on insects and small animals (which don't need to be chewed, just swallowed), having teeth became an unnecessary luxury, and they gradually disappeared over millions of years of evolution.