We all have areas that we like to cover up and those areas are generally fuller than we would like. Upperarms and biceps are a constant woe and not all patterns are made to accommodate this. If this sounds like you, check out my suggestions below on how to modify the pattern.
Step 1) Check the muscle of the body measurements and finished garment measurements to determine how much you will need to add to fit your arm comfortably.
Step 2) Measure the total length of your sleeve cap pattern. Remember this number as you will need to maintain it.
Step 4) When you need to increase larger amounts, the alteration becomes a combination of extending the muscle, but also reducing the sleeve cap height. By reducing the sleeve cap height the total cap length you measured is maintained.
Step 5) Reducing the sleeve cap height too much to match the cap length to the original length can be problematic. When increases exceed 1 1/2″ I prefer to modify the armhole to create a longer circumference. This will justify the longer sleeve cap length. To modify the armhole align all seams that comprise the armhole. Drop only the bottom level, starting with 1/2″ and reshape as illustrated. Blend to zero at points shown. Check this new circumference in relation to the required sleeve cap length and follow steps 3 & 4 as necessary.
As a side note, when the muscle becomes wider, the sleeve cap should as well, by small amounts (usually half of what you extended the underarm for each side.
This alteration is just a guideline as you will need to figure out how to make it work for your body, but hopefully it will give you the tools needed.