probably a number of possible reasons, I'll suggest a couple...
- after the big rides, you're ravenous and eating back the calories you've burned
- shorter rides maybe lower intensity and more like to burn fat
I've also read (I think it was greenbank of this parish) something like the actual weight loss doesn't occur until the week(s) following the big rides, something like the body kind of going into siege mode immediately following a big ride and hoarding in case you go and do it again, then the weight loss occurs when things have settled and the calorie burn needs/pattern is established