I'm a programmer and I sit by the computer almost all day long, some days literally all day, and I heard that sitting all day is one reason why someone's butt can get bigger is that true? and if yes, what is the science behind it?
Long periods of pressure (i.e., how your posterior feels after 9 hrs in the chair) have lately been discovered to decrease blood flow and lead to lowered metabolic activity in the individual adipose cells in the affected region. Some speculation but no conclusive evidence that reducing blood flow to the adipocytes (fat cells) decreases the rate at which their lipid stores are burned as opposed to well perfused cells. Hope that's helpful.
I don't see how that could be true in a "specifically increases butt size" way.
It'll cause issues in several other significant ways, including an increased likelihood of slower metabolism, weight gain, and a wad of other health issues, which have been discussed to death in the blogosphere of late.
Specifically butt-reshaping, in a statistically-meaningful way? Doubt it. If you're particularly concerned, try a different chair style (ball, kneeling, etc.) or better yet, start alternating with a stand-up or tread-desk.