You feel tightness under the armpits and see pulling lines at the front and/or back, running from the back of the collar to the bottom of the armpits.
To reduce the tightness in the armpits, making the sleeve width (bicep measurement) wider is the surest way. However, widening the sleeve width can also make the sleeves baggier and thus you will not be able to achieve the smarter look.
Alternatively, you may want to check that you have selected the right shoulder type. If you notice that lines are running from the bottom of the armpits up towards the back of the collar, it is likely that a flat shoulder type is more suitable for you.
Lastly, the tightness in the armpits can be caused by a chest width that’s too small. Widening the chest width will relieve the tightness and provide more room in the chest area.