After fat transfer to butt and hips. If you gain weight will butt and hips grow? (Photo)

Patients will commonly wonder what the effects of liposuction will be on their weight and more specifically their ability to gain or lose weight in treated and untreated areas. 
After puberty, an individual will have the same population of fat cells for the remainder of their lives (barring an intervention). Any changes in weight will manifest not as more or less fat cells but rather swelling or shrinking of those very same fat cells.
Liposuction destroys or reduces the population of fat cells in a treated area. These cells once gone are gone for good. The treated areas now harbor a reduced population of cells and consequentially have a reduced potential for weight gain or weight loss. However, a reduced potential does not make changes in these areas absolutely impossible. The remaining cells can still swell with weight gain potentially compromising a cosmetic result.
The final component is the least understood. This is that of weight/fat distribution. It remains unclear how the body directs particular populations to change. Patients will often lament that attempts at weight loss are frustrated by losses in areas not targeted and weight gain by contrast goes directly to perceived problem areas. It is impossible to predict how a patient will respond to weight gain. While we know that treated areas must theoretically have a lower potential for growth by virtue of reduced adipocyte populations, this does not necessarily always hold true with some patients always demonstrating the effects of weight gain in the same predictable areas. Similarly, post operative weight gain is not always manifested in untreated areas.
One way to think about this issue is in simple mathematical terms: if one has a population of 100 fat cells in a given area and liposuction is performed removing 50 cells-that treated area now has a 50% reduction in its potential to demonstrate weight gain (via swelling of the remaining cells); However, that treated area of 50 cells maintains a greater potential to demonstrate weight gain than another smaller area of 25 cells which has not been treated. Thus a treated area will have a reduced potential relative to it's pre-operative state but not necessarily to other areas (treated or not).
The best advice one can give is to reach a stable healthy weight pre-operatively and try to maintain post-operatively. This is the best way to achieve a good result with the best chances for longevity.
With regards to your specific question, fat cells behave the same regardless of where they reside. If you are prone to gain weight in one area and these cells are harvested and transferred they will continue to react in the same fashion to weight gain and loss.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

   In general, if you gain body fat those areas with fatty deposition should increase in size as well.Kenneth Hughes, MD

Yes, it is possible that if you gain weight, the buttock will enlarge a bit just like other areas of your body.