The only thing that I can think of that might have caused this growth is that I went into the project settings and turned off "optimize stills." I may also have turned on "maximum bit depth."
File size is dependent on Bit-Rate and on Duration. If you have upped the Bit-Rate, you will see an increase in file size. Will the Project still fit on your BD? If so, and you want highest quality, the Bit-Rate and Bit-Depth can just stay higher. If space is an issue, then you can drop these down a bit.
Something else to think about and look for is has your Duration changed. This can be caused by "Orphans," out beyond where you intend your Timeline to end. This ARTICLE will give you some tips. This is easy to check.
That was it! I had an orphan audio clip way out on my timeline. Once I cleared it, my project reverted to 9 GB.
Thanks a lot for your quick and expert help. You saved me a lot of money. Now I can burn copies of my movie on 25 GB Blu-ray discs rather than on 50 GB discs.
Those danged "Orphans" have bitten a lot of folk, as you can see - I wrote an article on it.
The most common cause is where one drags a lot of Assets onto the Timeline and then begins tightening things up. In a moment, the view out to the end of the Timeline is not there anymore, as one works near the beginning. Assets, or pieces of them, get left behind.
Glad that you found it, and cleared it.