Video with lower bitrate resulting from joining two video files
Your program probably re-encoded the resulting video file from scratch and used whatever setting it considered okay.

If you want to have more control about the process and/or do not want it to re-encode the video at all (if both are of the same format), then I strongly suggest looking into ffmpeg for this.

For reference, https://trac.ffmpeg.org/wiki/Concatenate - the concat demuxer is what you want.

As the most simple example, copy both your video files (let's call them videoA.mp4 and videoB.mp4) into the same folder. Create a videos.txt file with just this content:

file videoA.mp4
file videoB.mp4


Now run ffmpeg with this command:

ffmpeg -f concat -i videos.txt -c copy videoOut.mp4


If all goes well this gives you a new file with both videos in one file, with the original encoding intact.