Sparkle should not recompress images though, unless you have changed their settings (in a way that affects their pixels), or unless you clear the publish settings cache.
Hi Duncan, that’s what I was surprised about - in this case I just changed a single word, nothing else. - settings and pictures have been untouched for weeks.
The only other reason I can think of is low disk space, or system cleaning utilities, clearing out the Sparkle publishing cache. Sparkle is a well behaved Mac citizen so the cache files are in the system Cache folder, which the system does have liberty to clean in some cases (and tools like CleanMyMac or Onyx do overzealously).