Is your dog running beside you as you cycle? If so i imagine that he's suddenly using a whole heap more energy than he used to and that's where the weight went. the perfect weight is just being able to feel their ribs but not their spine. tho a puppy needs a little extra in case of growth spurts - he'll keep growing til he's two!
BEWARE: try not to let your dog run too much until he's 18 months to 2 years old as his joints aren't fully formed until that age. what ever he does just playing or messing around is fine but if you're out jogging / cycling with him you're pushing him further than he would naturally push himself. Just something to keep in the back of your mind.
Hope that helps.