Of course, if you keep your calorie levels low to begin with, the fact that you are eating carbohydrates alone will not make you fat. There are many skinny vegans and vegetarians out there who simply don't eat very much.
What carbs do have a tendency to do, at least in my body, is make me retain water. Because I had an Italian friend visiting a few days ago, I had pasta (a small portion) with clams, mussels, and cod. I also had a little bit of bread and a small amount of apple crisp for dessert. And sure enough, the scale was up 3 pounds the next day. Now, I know because I track my intake that I could not possibly have eaten enough for this number to be a genuine gain. So for the last couple of days I have concentrated on keeping my carb levels down.
Sure enough, this morning I'm three pounds down again. More importantly for me, I feel better. My stomach doesn't feel swollen. Days like that just drive home to me how much better my body responds when I'm feeding it the right kinds of foods: veggies, meat, nuts, oil.