Smaller paintings don't have as much detail and as a result the pixels are much larger (lower DPI). The picture frames have a higher DPI and the XL, XXL have the highest.
Cool, so I'll avoid the smaller paintings then.
Thanks for the help!
No problem. And you don't have to avoid the smaller signs, just don't paint a detailed picture on them. I personally like to use 2 huge wooden signs stacked and split my image on both.