I'm glad your Muffin is recovering. We had a big yorkie years ago. Like you my dad thought spaying was unnecessary as she would never be unsupervised in a yard and thought that female dogs got fat after spaying. This was back in 1975 and information wasn't available like there is now, let alone groups like this.
Cecily got really sick and she was diagnosed with pyometra.
http://www.marvistavet.com/html/pyometra.html
She required a hysterectomy and I don't think she ever really recovered from the whole ordeal. A year later she started failing and we had to put her to sleep. After she stopped eating, within in a week she looked like a skeleton with a head on it. It was the saddest thing, but we had to let her go. This time around, I had Debbie spayed as soon as she was old enough. I wasn't about to let this happen to another dog.
Stories like these need to be shared. People need to be educated so they don't lose their dogs needlessly.