If she can get by with a regular tampon for any length of time that is useful to someone who may have to leave the restroom when they are on their period then her menorrhagia must not be that severe. The sizes are based on absorbency. You can argue it's about preference all you want, and disregard everything else…

Actually as a nurse and a woman who has tumors in her uterus my advice is to tell your wife how lucky she is that she doesn't need an extra absorbent tampon. I have to use the 'extra large' ones because I bleed like a faucet every month. And even with the extra absorbent tampons and a pad I still have to make hourly…