This is a common concern, especially for those who really sweat the details of their daily intake and usual exercise routine. BTW, congratulations on discovering the key to maintaining weight and fitness: "Calories in vs. calories out!"
Although there can be many specific recommendations, such as Dr. Minniti's excellent ones, I'll trust that you are well-versed in both what is good for you as well as what your body needs and tolerates (not to mention enjoys). So let's stick to basics.
Healing after surgery requires a significantly-increased energy source, so even if your intake doesn't change much from your pre-surgical (with daily exercise) routine, your decreased activity levels after surgery are offset by the increased caloric needs of healing. In other words, if you decrease caloric intake while you are healing (to try to accomodate for being less active), your body doesn't have the fuel to heal properly. This is not trivial!
And if you feel that you will lose tone and definition while not exercising, realize that you can still be active, just not in periodic bursts of high-cardio burn. You can walk and move constantly (without raising blood pressure or pulse), burning calories and maintaining muscle activity, all while maintaining the consistent energy needed to heal without excessive bleeding, bruising, or even re-operation. Not to mention capsular contracture.
Specific things to avoid while healing after breast surgery are activities that involve the arms, especialy repetitive ones, such as elliptical trainers with arm motion, Nordic Track-like devices, rowing machines, etc. Treadmill use or jogging is best avoided at first (3 weeks or so--ask your surgeon) to minimize up and down movement of the breasts. When this is allowed, you should wear a supportive jog bra or two.
Realize that complete healing takes an average of 7 months, but you can safely return to daily activities right away, exercise activities in 3-4 weeks, and super-strenuous activites (kickboxing, contact karate, pole-vaulting, trapeze sex, etc.--just checking if you're really following me here) in about 6 weeks. As long as you return to your usual full-burn activities gradually, and according to your surgeon's recommendations (to avoid injury or damage to your new breasts), you will be able to do everything you did before surgery without concern. (Although you will also need to avoid tanniing beds or nude beaches for about a year until your scars fade completely.)
Also realize that thinking you "know better" than this ends up with you having a re-operation and doubling the time from your usual routines, plus the addition risk, cost, and reduction in likelihood of desired cosmetic result! Losing weight or exercising while trying to heal is not a good idea! Best wishes! Dr. Tholen