[Pansy-Mae]

It's probably been asked before, but what size on average should the mass under the nipple be, and should it be hard or squidgy.

I have quite a firm (sensitive) solid mass under both nipples, the left side is biggest, and about the circumference of a golf ball, maybe sightly larger. Is this normal.

I try to research this on Google, but everything I read is scary, and talks about cancer and ultra sounds, they talk about seeing a GP with any signs of Gyneacomastia. Has anyone else experienced this?

It normally first becomes apparent at about a large pea size and then slowly grows to about golf ball size. as it does so it becomes less bullet feeling and more firm but slightly squidgy.
Then it probably changes shape to become more like a little volcano than a ball and its quite normal for one side to be bigger than the other.
However not everyone experiences all of this in exactly the same way.

The references you've found almost certainly relate to advice to guys who have sudden and unexplained and unwanted gynecomastia...however, we WANT it and know why it is developing and are actively promoting it, so no need to worry about that, as such ( unless you are worried by what is happening in which case you shouldn't be doing this in the first place).