I understand what you mean with your reply, but the op needs to also realize that Tractor Supply gets their chicks from Hatcheries.
There is no pure Marans that come from a Hatchery.
Only from well known Breeders.
There are pure Marans from hatcheries as much as there are pure specimens of other breeds. They may not be the best compared to the standard, but they are still pure for their breeds.
However, certain hatcheries have made what is in my opinion a completely deplorable decision to produce hybrids using two different breeds and still give them names of pure breeds instead of calling them something else. Midnight Majesty Marans is one of those hybrids, produced with one Marans parent and one parent of a completely different breed, and it unfortunately only leads to confusion.

So will my lavender Orpington roo still be big
He more than likely will be bigger than your hens, of course, but not as big and especially not as fluffy as a proper Lavender Orpington from a breeder.