Yeah there are probably a few more ways of doing this but you're probably best off with find as it's so tunable.

find /path/to/dir -size -100M | xargs tar cvf archive.tar

or

tar cvf archive.tar $(find /path/to/dir -size -100M)

If there are other directories in the current folder which you do not want to archive, then use:

find /path/to/dir -maxdepth 1 -size -100M | xargs tar cvf archive.tar

Per the comments below, spaces can throw out some errors but in my testing they still seemed to get added to the archive when using the first method (a little strange in itself) but if you're worried, you can fix the find command to replace spaces with escaped versions.

As ever, there are a billion ways of doing this.

find /path/ -size -100M | sed 's/ / /g' | xargs tar cvf archive.tar



find /path/ -size -100M -exec echo '"{}"' ; | xargs tar cvf archive.tar