Can’t speak to this question specifically- but I do know that at MK - when I had multiple popcorn buckets and they didn’t have a locker available- they stored them in their ‘virtual’ locker (basically space where they keep wheelchairs, ECVs etc) for the same price as a large locker.
Keep in mind that while EP and MK offer the Jumbo sized lockers, park entry rules have a smaller limitation of 24′′×15′′×18′′.
It might seem like a contradiction that the jumbo locker allows larger than park entry rules, but that is because people with multiple items might still find use for the jumbo locker.