Overalls

VR
At the beginning of my junior year (August 2015), I bought a pair of overalls. I range from a size small to medium. However, when I put on the overalls, my butt would not fit into the mediums. I grabbed a large and tried it on. It's a bit tight around my thighs and butt, but I'm 5'1 and the buttons to connect the back part to the front hang too low to connect. I haven't been able to wear them for this reason. 
Short: Size large overalls do not fit on top half. The buttons hang too low to connect. 
Any tips on what I could do to make them fit?