Ah right - you're knitting a yoked jumper. TBH, I'd just continue on up - I doubt that a difference in the upper arm matters too much there.
You may need to watch your stitch count. When you have both sleeves and the yoke on one needle, you may have less stitches than the pattern says you should, because of those smaller sleeves... But the bottom line is, you need to keep decreases where they are meant to be as you follow the pattern. (Usually decreasing around about 4 places round the whole thing; either side of each sleeve). So just check the maths; make sure you are continuing to decrease where it wants you to and make sure, if the yoke has a pattern, you won't bodge it up with a slightly smaller stitch count. I'd probably continue to follow the pattern for Large, once you're on one needle, but just take into account how many stitches you are short.
If you're only a few stitches out, I wouldn't bother. If the discrepency is large, you may need to re-think.
Another way is to unpick the arms down to about elbow level, and increase at a less dramatic rate, so by the time you hit the point where they go on one needle with the body, you have the correct number of stitches.
You can do the maths by measuring how much more you'd need to knit from, say, the elbow (roughly half way point in terms of length of sleeve) - how many rounds per 10cm. Then take the final number of sts you'd need when you hit the top of the sleeve, before it joins the body, and minus from it the number of sts you have at the elbow, currently. You're probably increasing two stitches in a round on an increase round. So then half that number. This tells you how many times you have to increase over the number of rounds you have left. Then you can space the increases accordingly. Bottom line is - it's not entirely crucial when you increase up the sleeve so long as it's consistent and you end up with the right stitch count at the top of the sleeve, before it joins the body.
Hope that makes sense.