After breast augmentation the implant should be centered under the nipple, and fit the diameter of the breast. The limit of course is the chest size. What we like to see in an attractive breast is a pleasant 'S' curve extending from the underarm, back to the chest muscle, and then out again along the side profile of the breast, and then back to follow the fold under the breast. The curve is a very soft one and the implant should not rest past the line from the front of the underarm down to the hip.
An implant too wide may risk running the breasts together in the middle, or cause the implant to sit out past this line onto the side of the chest or underarm area. Many women after breast augmentation will experience the sensation of touching the breast with the arm as the arm is brought forward or across the chest. This is very normal, what women with a fuller breast experience all the time without thought or question. If your implants are indeed too wide there may be an issue, or if the implant is too large for your comfort or activity level you may have an issue, otherwise the feel of the breast against your arm is quite normal.
Best of luck,
peterejohnsonmd