Classic and Slim: choosing the right Minecraft skin model
The difference is in the arm layout, not the size of the whole PNG.
One image size, two arm shapes
A modern skin can use a 64 by 64 pixel image with either a Classic or Slim model. Classic arms are four pixels wide and Slim arms are three. Choosing a model affects where the arm texture appears on the character; it is not a command to resize the entire image.
Why sleeves can look wrong
Imagine a stripe placed at the edge of a sleeve. When texture regions do not match the selected model, that edge may appear on the wrong surface or leave an unexpected strip. First check the model instead of immediately repainting the sleeve.
Check more than the front
Look at the outer arm, inner arm, shoulder and underside. A front view can hide a seam that becomes obvious when the character moves. Keep the original PNG while experimenting so that a model test does not overwrite your only copy.
Match the choice when importing
Where the game offers a model choice, choose the model for which the texture was made. File dimensions alone cannot tell you that every arm pixel is correctly mapped. Compare the result in the game as well as in an editor preview.
A simple diagnostic
If the head and torso look correct but the arms do not, model mismatch is one useful possibility to check. If the whole image is scrambled, also verify that you downloaded the skin texture rather than a preview screenshot. These are different files even if both show the same character.