Our pelvic floor muscles (PFM) play a big part in helping keep our bladder closed so urine doesn't accidentally leak out when we aren't ready. Sometimes, even when it's not full, our bladder will squeeze really hard. If your PFM are weak, you might leak some urine, this is called 'urge incontinence'. If they are very strong, you might just feel you need to pee more than usual.
A well-functioning bladder should be relaxed while it fills. It sends a signal to the brain when it is getting full, but does not squeeze until you are sitting on the toilet and your brain sends the right signal. Then wollah, pee on demand!