Without knowing exactly what is under the "hood," my first guess would be that a different bit-rate was used, one choice vs the other. A simple bit-rate change could account for a significant size difference in the Transcoded material.
Yes,the bit-trate would affect the file size but why more when the "fit contents to available space" is checked?
Should I leave it checked or unchecked when my bluray project is small enough because it says the bit-rate is higher when checked (24Mbps checked as apposed to 19.19Mbps unchecked)?
Why that bit-rate would change, when one is no where near the max Duration for a BD is beyond me. I would go with the max bit-rate for the ultimate quality.
Good luck, and maybe others can tell you the "whys" of this behavoir.
Default bitrate is 20.19Mbps for 1920x1080 settings in this preset. It calculate the size based on 20.19 bitrate and the maximum bit rate setting is 24mbps bitrate for this preset. If we check the "Fit content" option then PRE sets the bitrate to maximum(24mbps). if we add the contents(more than 25 GB) in timeline and check the "Fit content" option then it will reduce the bit rate to fit the content in 25 GB Disc.