[FFmpeg-user] Joining mp4 files did not yield the expected file

[jd1008]

Command:

ffmpeg \
-i Discourse_1_Part_1.mp4 \
-i Discourse_1_Part_2.mp4 \
-i Discourse_2_Part_1.mp4 \
-i Discourse_2_Part_2.mp4 \
-i Discourse_3_Part_1.mp4 \
-i Discourse_3_Part_2.mp4 \
-strict -2 -y All_Discourses.mp4

Yielded a result much smaller than expected (in size) and
when played, does not contain the contents of all the parts.

$ ls -la
-rw-r--r--. 1 jd jd 122410407 Dec 29 11:25 All_Discourses.mp4 <<<<<--- 
The output file
-rw-r--r--. 1 jd jd 179667977 Dec  1 10:20 _Discourse_1_Part_1.mp4
-rw-r--r--. 1 jd jd 269749812 Dec  6 14:38 _Discourse_1_Part_2.mp4
-rw-r--r--. 1 jd jd  66092606 Dec 12 18:03 _Discourse_2_Part_1.mp4
-rw-r--r--. 1 jd jd 113322556 Dec 12 18:17 _Discourse_2_Part_2.mp4
-rw-r--r--. 1 jd jd  47993972 Dec 12 18:34 _Discourse_3_Part_1.mp4
-rw-r--r--. 1 jd jd 157472297 Dec 12 18:25 _Discourse_3_Part_2.mp4

What am I doing wrong?