A lot of it depends on whether her tits are real or fake.
Really big implants tend to make tits overly round and firm. When you fuck them, the tits don't have enough "give" to fully wrap around your dick. There's plenty of pressure against your dick, but it's mostly coming from only two sides rather than from 360 degrees all around. So tittyfucking big implants feels more like a handjob than a pussy or mouth. Your dick also tends to slip out of the cleavage a lot so it's harder to maintain a good thrusting rhythm consistently.
Really big naturals on the other hand are much more pliable, so you can easily mold them into a sheath that completely surrounds your dick. With the right amount of lube it can feel pretty similar to a mouth or pussy.
The only real downside to fucking gigantic natural tits is that they tend to be really heavy, so her (or your) arms get tired after awhile from holding them upright or moving them up and down. This is more of an issue for titfucks when the girl is in the kneeling or leaning over you position. Less so when you straddle her torso and fuck her tits.