Poser does not scale anything. A .jpg or .png image bears no memory of its history so Poser would have no information to go by. Poser expects the u and v coordinates to have values between 0 and 1. If the value provided is outside that range, it ignores the integer part. u = 3.234 is taken as u=0.234. If your texture is saved as 4000x4000 it simply takes the rgb value of the  936th. pixel at the given (v) row.