The not-oft seen naughty side of Pluto Planitia.

She's usually far too busy with her adventurous life, but in the slices of reality where she possesses chestbreasts, she'll have moments in private where she'll let her family inheritances stand freely.

Pluto carries the Hyper gene, and in her family, that gene is expressed through hyper nipples. Her teats are indeed, quite large, but among the other mares in her family, they are some of the smallest, and she remains insecure about that. 

Medical technologies in the future could allow them to be enlarged, but she feels that would be cheating, plus, she already has to pay a premium for the specialty titanium-weave bras she has to wear to keep them contained for her action-packed lifestyle. Going and GETTING bigger nipples would be too much of a hassle, all told.

Still, she can't help but wistfully imagine that someday, somehow, they'd grow even bigger. Maybe even bigger than the rest of her family...

Consider this to be canon to any depiction of Pluto that has chestboobs.