Looks to me like they expand as the nut is tightened...
You can try partially expanding them outside the bars, until it just barely slide in,... and then tighten to snug...
If they just won't expand enough... (like my bar-end mirror wouldn't),... just stuff something into the bars to take up some space...
(I tried several things... I think I ended up with some old innertube rubber. I can't really remember)...
(or it might have been wood shims)...
...
The point is,... to fill the space...
It seems hokey while yer' doing it,... but, once you get it snug, you'll forget the hokeyness and it'll be just dandy...
...