it is the effect of carbs: carbs->sugar->insulin-> fat

insulin is a powerful hormone which, among other things, sends a message to the body to store more fat instead of burning it

a sample size of 1 is hardly significant but you could be far from fat because of a number of reasons: you are young and/or have a high metabolism, you are active, your genes, you eat other foods which counter the carbs in pasta (fats and fiber), etc. there are a lot of variables

the ideal diet is one where your blood sugar is more or less constant throughout the day

the worst diet is one where your blood sugar spikes, forcing your pancreas to release insulin again and again