Does having a capsulectomy cause your breasts to be smaller?

Much will depend on how thick the capsule is and how carefully the procedure is performed. For example, if the capsule surrounding the breast implant is very thin and care is taken to avoid removal of any non-capsule tissue, the  patient's breast should not be any smaller after breast implant removal surgery.  
Generally speaking, what your breasts will look like after explantation depends on several factors such as: the quality of skin elasticity (the better the elasticity the better the skin will bounce back), the size of the implants used (the larger the implant the more trouble you may have with redundant skin), and the amount of breast tissue present at this time (which may have changed since the time of your breast augmentation).
Life experience since your breast augmentation procedure, such as pregnancy or weight gain weight loss, will potentially influence the factors discussed above. If you take these factors into consideration and apply them to your specific circumstances you may get a good idea of what to expect after the implants are removed.
Patience is definitely indicated after removal of breast implants; it will take many months for the skin to “bounce back” and for you to see the final appearance of the breasts.
 
Best wishes.

In most cases, removal of the capsule does not require a lot of tissue removal.  The scar capsule is separated from the muscle/breast tissue in such a way to remove only the capsule.  Removal of the implants themselves will change the size of the breast. Removal of the capsule itself does very little to change the size of the breasts.  Be sure to discuss all of your concerns with your plastic surgeon.  Best wishes.