“Your Big Fat Boyfriend,” a new book by Ithaca College alumna Jenna Bergen, reads more like a chat between girlfriends than a book on dieting and exercise. Inspired by her horror the day her “totally hot, erase-10-pounds, make-my-ass-look-amazing” jeans began to expose an unsightly “muffin top” of triple chocolate chunk proportions, Bergen weaves together anecdotes…