I think there's two good reasons to reduce the size of the buttons for larger displays.
First, really big buttons on a bigger screens can be overwhelming and take over the visual hierarchy of the page, this can be distracting to the user. Also, they are not a common or familiar practice on most web apps and web sites so big buttons seem unfamiliar or strange to users of larger displays - they seem out of place or like they don't belong. Distracting.
Second, bigger buttons don't always mean better. UX matters published a great article on designing for touch screens. While this article is written about designing for touch they make some valid points about making buttons too big and designing visual targets - I recommend reading it. I the article, they make this point:
If buttons get too big, users start having problems perceiving them as
clickable buttons—and sometimes even discerning them as discrete
There is also a study from the Human Factors and Ergonomics Society around building and designing computer workstations that might have some standards or guidelines - 100-2007 Human Factors Engineering of Computer Workstations