How to add sizes of all files with the same name in UNIX Shell scripts

I'm new here and I want to ask how to add sizes of files with the same name? My file names are actually different from each other but I cut them so I could get files that are grouped together.

here are my original files with sizes (sample):

1. 12345 a_1.txt
2. 12234 b_1.txt
3. 32123 c_1.txt
4. 11122 a_2.txt

Now I cut the filenames to remove those characters starting from '_' (underscore). They become like this:

1. 12345 a
2. 12234 b
3. 32123 c
4. 11122 a

Now I want to add sizes of all files with same filename as seen above. Output should be like this:

1. 23467 a
2. 12234 b
3. 32123 c

Please help. Thanks a lot guys. I'm stuck here for hours now