Bloodgood is one of the larger growing JM's and may just never attain its potential in that limited a growing space. Roots are not an issue with JM's in that they are not invasive or will not cause disruption but they will certainly fill that space in time. Limited root space always results in stunting of growth.

There are other red leaved JM's of smaller scale that could work equally as well and be happier for a longer term without attention - check out Fireglow, Pixie, Shaina, or any of the red leaved dissectums. While these may not grow as large and imposing as the Bloodgood, they will make a statement - the laceleaf maples in particular can have a very big impact for a not very big tree.

Also not as likely to get stuck with an inferior seedling (instead of a true-to-type grafted plant) under one of those other names.