@Loki The Flameshield reasonable question. I have thought of a few possible explanations. The simplest being he/she was genuinely that unobservant. But while such people do exist, this is highly unlikely do to him being aware of his own fatness.
A more complex possibility would be that he/she is spending every single day doing absolutely exhausting workouts (not recommended). As a result he just goes home and sleeps then begins the exhausting exercise again. Giving himself no real time to be cognizant of what is happening. Add in the perfectly normal disbelief that a drink can change you, not being able to see below the belt from a standing position, and all the rush of hormones; you have a recipe for enough cognitive dissonance to not notice until an outside force actually engages with her, forcing the disillusion of her cognitive dissonance and address her current condition.
The lengths the human mind will utilize to protect itself and/or accomplish a strongly held desire is quite miraculous.