This is a widespread problem, telling the Greater and Lesser yellowlegs apart. A problem from Alaska to Tierra del Fuego and beyond. It sometimes seems like there is an "intermediate" yellowlegs, as our focus on size can be unreliable when a bird is by itself. Plumage differences do not help much. The key is shape and structure, bill size and color, as well as neck shape. I hope this helps!! Get out there and try out these features.