As JMSAVOYAT says, it will be interesting to know if your piece has multiple configurations and the minimum size you get when you make a file saved as.
Personally, I have also encountered a problem of this kind in the event that a display state is "corrupted", so additional question: do you have display states related to the configuration or not?
Make a copy of your file and go up the function tree to the 1st function and go down to the function that will take the longest to open, delete it and see if the size of your file is back to normal. This happened to us for a 13 MB file that became 270 MB.