I observed this only last night! I have two particle systems running and what was a half MB Blender file before the particles were added, suddenly increased to some unbelievable size! (can't remember, but it was huge!) I then switched on the caching and nominated a spare folder.
No baking is involved and when running it, the particles ceased to appear, BUT - the filesize when resaved returned back to 600 KB. I cleared everything including Blender's temp buffer, caching folders etc so when it was reloaded there wouldn't be any hangovers.
The particle settings I'd made were still intact, so I switched off the caching and everything returned as before, particles and all!
My procedure now for this particular project is, prior to a final save, tick the caching box for EACH EMITTER, then save. When reloading, switch it off again. No more humungous filesizes!
Note again - for this to work properly you must nominate a folder for caching.