That depends entirely on how tight you pants are! I did not have to buy larger pants. My preference is IWB because I can still dress professional and discreet... so i carry IWB all the time, just so I don't have to invest in more gear for off days or other clothes for work.
The topic of comfortable carry rarely translates accurately from one person to the next... unfortunately, it's a trial and error process. For a new guy carrying, IWB can be hard to get used to... part depends on the size and thickness of gun, holster/belt combo, belt position, cant and whether or not you wear an undershirt.
What gun do you want to carry and are you R or L handed and what's your normal dress code? Best option would be to see if you can borrow or try out a holster or two just to narrow the style or direction you want to go. And you may get lucky and like a specific set up from the start. Anyway, the more info you can provide, the better the advice we can give.