In the end, this is a good thing. However, I believe the increase is sarcoplasmic - superficial muscle if you will. I don't like the term, as it does have a purpose (can hold more glycogen, for example) - Still, if you quit working out, you would drop back down to your normal sized arms. It may take month or two, but they would return to normal size as you haven't been lifting long enough to have gained any true size to the muscle.
When people talking about losing LBM or gaining it (especially in relation to a cut) take little stock in that. While technically LBM is anything not fat, most people seem to think it means muscle. That really isn't the case. One can almost always assume that 10 pounds of lean mass in either direction isn't a true loss or gain and is based primarily around training and eating habits, which, if changed, will also change your lean mass weight.
That said the bicep itself can't be measured with a tape measure... I think you mean to say your arms have increased in size. You have a tricep component in there.
Keep lifting!