We would suggest going UP a size if you do not like tighter fitting sweatpants 

Things I’ve done in these sweatpants (because they’re THAT comfy)

• Jumped a very excitable horse
• Walked around the place acting like I wasn’t wearing horse riding clothing
• Worked out in them
• Slept and lived in them for 3 days while sick with the flu

Model is 5'9, commonly an AU/UK Size 6 and is wearing an xxs