Xtol does have a low capacity limitation; Kodak says to use at least 100ml stock per roll, whatever the dilution.(posted 8992 days ago)Xtol does have another problem; it appears to be rather sensitive to contaminates in the water used to mix it, plus even if distilled water is used it may still be rather prone to aerial oxidation.
Also, Kodak had and may still have problems with the packaging of part A; at worst it's caked solidly, and freshly-mixed developer is dead without any color difference to indicate so.
I haven't had any problems at all with Xtol, but there have been many reports.
So...although you used more than the minimum stock, if the developer was weak it may actually not have had enough strength.
The result of that would be thin negs, which you didn't mention. It wouldn't cause an increase in graininess. _Printed_ slightly thin negs would make grainier prints, assuming you'd use higher contrast to print them.
Water quality, though, can cause significant differences in graininess. Here's a story; some say it's impossible but it happened to me.
I was happily using Rodinal with TX many moons ago; sure it was a little grainier than D-76 but it showed very high acutance and a nice tonal rendition. I moved to another city and suddenly had _golfball grain_.
Suspecting the change in water, I used distilled water to mix the developer. Still, golfball grain. Then I mixed the developer, stop and fix with distilled water and _still_ got golfball grain.
Frustrating.
Ed Meyers at Pop Photo suggested I call a certain chemist at Agfa, which I did. The guy immediately told me that the problem was "hard" water and said that the installation of an ordinary commercial water softener would solve it.
I had Culligan hook up the basic tank....FINE GRAIN!
The point is...get this...the water used to _wash_ the film was having a tremendous affect on graininess. Weird but true.
Water-quality managers add all sorts of stuff to water to make it safe, and that mix can change by the seasons. So, if you haven't already tried it, at least mix the developer with distilled water. That might make a difference.
Also, Xtol is pretty grainy compared to, say, D-76, and the apparent graininess changes with dilution. In my experience with the films I use, straight Xtol gives the finest grain, least speed and lowest acutance. Xtol 1:1 is the sweet spot, and while more dilution works, it's sliding down the slope of increasing graininess.