Ariana Grande boob job rumors were sparked after a noticeable change came in her bra size. Her breast size is only listed as 32A, but recent photos suggest she's now more of a B cup. You can see that she's pretty flat chested in the before picture, but is undoubtedly more busty in the after picture.
The Ariana Grande breast implants gossip didn't come about until she was about 19. It wasn't until then that her bra size increased by a cup size. She's never admitted to getting a boob job (obviously), so all we can do is examine photos of her around age 18 compared to now (22). Seeing as she was fully matured in the before picture with a noticeably smaller bra size, one would think she's gotten a boob job. Just like her alleged nose job, the breast implants would have been very subtly done. You would never think she had any work done if you hadn't seen the before pictures. Everything about her looks completely natural. You can see more Ariana Grande plastic surgery here.
Her bra size looks much bigger in the fourth picture VS the first one. She could very well be wearing a padded bra in the after picture, but there's significant enough of a difference to assume it "not" natural. Once again, Ariana Grande hasn't admitted to getting a boob job, so it's up to you to decide!