The sleeve cap is the curved top section of the sleeve that fits up against your shoulder.
Sleeve caps are usually symmetrical and armscyes are, too. But this useful image below shows that this symmetry doesn't mean the sleeve sits symmetrically on the shoulder. Look at the angles at which the shoulder and side seams run - they're different - and how the back half of the armscye fits differently from the front half. Because we are working with knitting fabric that stretches, these differences are easier to work with, but the principle is still the same.
When upsizing patterns, I often need to lengthen my armscye to give me room to make sleeves larger. For example, if a pattern has sleeves that are 17 inches wide at the largest point, I know that I need to make the sleeves two inches wider to accommodate my 18.5 inch biceps (1.5 inch difference plus .5 inch for ease equals two inches wider). Assuming I'm knitting from the cuff up, I adjust my increase rate so that my sleeves end up two inches wider at the widest point. Then I bind off those extra stitches in the next few rows of decreases used to shape the bottom of the sleeve cap.
If I make the sleeve wider, I also have to adjust the armscye opening so that the sleeve fits into sweater well. So I increase my armscye length by one inch on both the front and the back, thereby getting the extra couple of inches I need. I place this adjustment in the middle of the armscye after the decreases for the sleeve hole opening.
These modifications give me a great fit every time. I do make other mods that improve my sleeve fit, but I'll save that discussion for another day. Until then, carry on, intrepid knitters! Keep making your own sweaters that fit.