She's referring to the flabbiness (bits or rolls of fat, though you don't say that) or a belly perhaps that would bounce around/wobble if you ran in less tight clothes. 'Wobbly bits' on the body could refer to any part of the body that can 'move' ... it all depends on context. Generally they mean the flabby parts, rolls of fat .... big or small .... but other parts too depending on gender and context. BUT, because she's using the words 'suck in' that implies mostly the belly. For instance, you often use the words 'I need to suck in my belly/stomach ....' when I want to appear with a smaller stomach in a photo.

I'm Australian. Without knowing the context of your quote, "wobbly bits" could mean three things in relation to a person's body. 1. any excess fat on the body, eg. belly fat. 2. a woman's breasts. 3. a man's genitals. The running gear provides support so these parts of the body do not wobble, ie. move about excessively, while running. This usage is quite informal and rather picturesque! I'm not aware of ever having heard this term used in US media. I think I have come across something similar in UK media. Hopefully people from these countries will contribute.