Quote Hi,
I am just starting to work with Power2Go... and already stuck with a problem:
The mp4-clips, which I want to burn on a Verbatim 50GB (double-layer) Bluray, amounts to 34,41 GB total size, so they should easily fit on that disc.
However, Power2Go "explode" them to tremendous 287,27 GB project size: What?!?!
I already tried to adjust the Disc Presettings to conform with the parameters from the original movies, HD 1280x720/50p (26 MBit) but that did not reduce the project size at all.
If I will not be in a position to "convince" Power2Go just to "copy" the source material, the program will be absolutely worthless, anoying waste of money!
Many thanks for every support!
(I already published this enquiry in the German forum, regrettably in vain)

You already depicted all of these as the faults power2go made.
But, I'm wondering you didn't know the difference between a "Blu-ray Video" disc and Blu-ray "data disc".

If you want your MP4 files to be burned on a Blu-ray blank disc without any conversion, I suggest you click the Data disc > Blu-ray option, but not create a Blu-ray Video disc using another Video disc tool.

Data disc tool will back up your files on Blu-ray disc like what you did when copying files to an USB thumb drive.

And, of course the burned Blu-ray data disc is not compatible with a Blu-ray player that does not support playing standalone MP4 video file (on the disc). It will be read-able and playable only on PC platforms with BD drive using a player software, or a player hardware that supports playing standalone video files.

If you really need to create a Blu-ray video disc from the MP4 files, the standard Blu-ray video disc's encoding compression and efficiency are terribly lower than modern MP4 or MKV video files, which are encoded with H.264 or H.265 formats.
The produced video size on the disc will be much larger than original video files and the video size (on the disc) will be affected by the time length of your source video files only, but not the source video file size.

The video transcoding/conversion is required and un-avoidable when you're producing them into a "Blu-ray Video" disc that are universally playble on Blu-ray certified TV players.