@gladiolus-the-batty: This is so wonderfully wholesome. As for the question: How big is too big for a plump pone?

Tip #9: While it might be tempting to make your pony as big as you can, ponies will have different tolerances for different weights! While you might get lucky and find a pony that will be perfectly happy being your immobile love generator, it’s not uncommon for a pony to want silly things like “mobility” and “to not weigh a literal ton.” Don’t worry, though! Keep working with them and watch their waistlines creep outwards!
Mine still insists on being able to work- working on fixing that!