Hello. These are a loose fit pant so you can certainly wear padded cycling shorts
or padded liner shorts
under them, yes. You should not need to "size up" to be able to layer under them because they are designed to have a looser fit than your snug-fitting cycling shorts do.
Please let us know if you have any questions and we'd be happy to help: 1-800-783-8326 or email@example.com.