The health app doubles in size after every single iCloud backup. I believe it's a bug. Instead of only updating the changes, it adds everything up again. Delete your iCloud backup and create a fresh one. If you go to settings and check the usage of the health app, that is the size that it will start as with your iCloud backup. But every night when it auto backup, it will double in size again. I delete my iCloud backup and create a new one once a week.