As a Spanish company, the sizing of Joma uniforms is a more "European" fit and different from what we are used to with our previous jerseys. Our "Legacy" style, older jerseys were by High Five (Galactic "Tiger Stripe" purple jerseys) and Sport-Tek (alternate/practice Gray or the older solid purple mesh jerseys). The High Five sizes are a little smaller than Sport-Tek, Joma is significantly smaller than High Five. In general, the Joma Fit is almost a full size off - High Five Youth Medium is the equivalent size of Joma Youth Large, etc. So if you are looking to maintain the same fit, you should move up a size. If you are looking to get "the next size up" for your growing player, you should probably move up two sizes from their current High Five jersey.