I was told by LC that it's normal to have differing volume from both breasts. I remembered she told me something about 1/3 from 1 breast & 2/3 from the other.
My best advice is.. RELAX! dont't keep staring at the bottle (this is where the handsfree kit is really useful if you are using electric pump ) cos it'll just get you stressed out about the vol expressed.
I won't say that's engorgement, more likely you either have blocked ducts or the milk is not fully drained. Do massage the breast before you continue pumping.