胸大不是天生的就是揉大的
[xiōng dà bù shì tiān shēng de jiù shì róu dà de]
The phrase is somewhat vulgar. It means that having large breasts doesn't always depend on genetic factors, but might result from other influences. Such internet usernames may reflect people expressing their personality openly, although this name sounds a little tasteless. Of course, such words would not appear in polite occasions and should not be used widely.