Me thinks they let their stomach's do the portion control, in times of plenty, but as mentioned sometimes there wasn't plenty.
That's one of the reasons I like the Intermittant fasting (IF) protocol, not a strict rule, but as a general principle.
By restricting the feeding window, 2-8hours, depending on what suits you, most days I only have one meal, sometimes two, and sometime a little bit of snacking around the place. Rarely ever eat before noon and usually my main meal is around 6pm.
The IF allows you to really get in and enjoy a fully satisfying meal, rather than endless tit bits, that never fill you up, but just keeps insulin and glucose levels elevated, there are many health benefits to IF, but like anything with the Primal life, if you need to set timers and stop watches to stay within the guidelines, then you're not really being primal, there are no clock's on the savannah.
I looked at a lot of indigenous and evolutionary data to just try get a feel of how our ancesters may have lived their lives, and with the abscence of refrigeration and agriculture, IF would likely been a part of normal everyday life.