I agree, if given plenty of exercise and optimum temps, it would likely be difficult to overfeed juvenile monitors of most species, but I've seen plenty of obese adult sized captive monitors, particularly noticable when compared to their wild counterparts, so something's going horribly wrong somewhere? I suspect some keepers continue to feed larger animals at hatchling/ juvenile rates (an all you can eat diet) rather than reducing intake accordingly. Part of the reason why "environmental enrichment" options should be a consideration when aquiring larger species of monitors, to avoid boredom and associated inactivity.
Many odatria likely reach breeding size in the wild within their first year, so this is occuring naturally anyway. Gillens immediately spring to mind.