i wasnt aware there is more than one size of MTB barrel-style cable end. I got these two cables from different bicycle dealers online.

The top one is what i consider to be normal size MTB.

The lower one is a tad smaller. The wire diameter is smaller also.

i didn't try to put the smaller one into a lever, but it looks like it might still work.

The smaller one may be from a different application, possibly small engine throttle cable, like I found in this photo:

The combination of a smaller-than-normal head size plus the smaller-diameter cable would make me leery of using it in a safety-of-life braking application. The head also looks to be somewhat poorly molded with a lot of mold flash. A derailleur cable parting is an annoyance but a brake failure is a whole 'nother animal.

Will a "standard" brake cable's head fit the lever? If so then the worry is misplaced, just install a current cable and be done with it. If not then the lever manufacturer might know something we don't

Over the years there's been a number of cable wire and end shape dimensional differences. Remember the early Shimano MtB cables? With their larger heads, or the tiny brass curved shims to fit a more common cable to their levers? Vision Tech uses a brake cable wire that's both slightly smaller in diameter with a ball end shape. Campy's brake cable wires for decades were bigger then the current 1.6mm "standard. Andy

Just FYI since I don't know specifically what you got. Shifter and brake cables used to be pretty much the same diameters back in the old days. But with indexed shifting and other things coming into the picture, shift cables became thinner diameters.

So if you buy bare cable, be aware that it might not fit another housing made by some other manufacturer.