Spine tries to be smart when a mesh image changes size:
If the image is larger, Spine assumes whitespace was added to the edges, so it keeps the UVs the same size, centered by how much larger the image is. You may need to use Edit Mesh mode to move all the UVs. If you scaled the image up, keeping the UVs the same size isn't what you want. Spine doesn't have a way to scale the UVs, so you'll have to redo your mesh.
If the image is smaller, the UVs are scaled the same amount, which would be what you want if you scaled the original image down. If you trimmed the image, so the image contents are the same size but the image dimensions are now smaller, then the scaled down UVs won't be what you want. Spine doesn't have a way to scale the UVs back up, so you'll have to redo your mesh.
One solution would be if Spine prompted when it detects a mesh image resize to ask what you want to happen.