I have split my favourites old leggings from Next which they no longer do the same style. They had a massive rear pocket, kind of had a fold over hidden flap at the top of the bum. I assume pocket is for phone and keys but I could fit 3x tennis balls in the pocket.
The leggings were never advertised in the description as having the pocket so I have no idea of any of the current next gym leggings have it. Or indeed any other brands?
The pocket is much better than smaller leg pockets where you can only just slide a phone into it.