Mine have been losing weight recently, but eating well. I've been keeping a close eye on this, thinking "thyroid probs!". But they are more active, chasing round the house, though they don't go out much cos I have to keep them out of the sun. And they're eating like piglets!
Glad to hear this is normal in the summer. Polly always lost weight in the summer before Mrs Vet put a no on any sunshine. But Samantha tends to be a lazy cat (says me: they're chasing round the house as I type).