The best estimates of peak Neanderthal population are about 70,000 (census population, not effective population). This translates into roughly one Neanderthal for the entire duration of the species for each modern human alive in their former range today.
Homo erectus was undoubtedly less impressive than Neanderthals in almost every way. They had less advanced tools, particularly in Asia (although some speculate that the lack of stone tool evidence could be due to the development of bamboo tools that weren't preserved). They were smaller. They had less language ability. It isn't obvious that they were apex predators.
Given that more advanced modern humans appear to have had much higher population densities than Neanderthals, even at the hunter-gatherer stage. One recent estimate comparing Neanderthal and modern human strata at the same location in immediate succession put the modern human population density to the Neanderthal population density ratio at 9:1 or 10:1. Given all of this, it isn't unreasonable to guess tht Homo Erectus might have had lower population densities even than Neanderthals. The ratio might be a factor of two, or less, and probably wouldn't be more than the factor of 10:1 of modern humans to Neanderthals.
The size of the geographic range of Homo Erectus in Asia (and by Asia in this context I mean to the east of India) was on the same order of magnitude as the size of the geographic range of the Neanderthals.
So, the census population size of Homo Erectus may have been on the order of 7,000 to 35,000 over this rather vast territory, at any one time, broken up into perhaps 700 to 3,500 separate small bands, no larger than the modern human or Neanderthal bands, and quite possibly a bit smaller as weaker communication skills may have limited their group size.
Now, Homo Erectus was probably in this part of Asia about ten times as long as Neanderthals. But, from a genetics perspective, this may have caused them to be mutationally limited. The number of new mutations in the total population is a function of the number of births that take place. Smaller populations produce fewer mutations. Larger populations produce more mutations. There is no good reason to think that the number of selectively beneficial mutations isn't relatively fixed relative to the total number of mutations. So, Homo Erectus may have had a similar number of selectively beneficial mutations over their 2,000,000 years in Southeast and East Asia and the vicinity as the Neanderthals did in their 200,000 years or so in Europe and the vicinity.
Moreover, one might actually expect that balance to favor the Neanderthals. The likelihood that a new selectively beneficial mutation that has not reached fixation will be lost in a population slump is greater when the population is smaller than it is when the population is larger. And, the likelihood of new mutations having benefits for the Neanderthals, who were living in an environment quite different from that where hominins evolved in Africa, may have been greater than for Asian Homo Erectus who were living in an environment closer to the one that their African ancestors were already optimized to live in through millions of years of selection. The better a fit one is to one's environment, the less likely it is the a new mutation will provide selective benefit relative to the non-derived part of the genome.
Thus, we might expect that Homo Erectus in 2,000,000 years in Asia, with a lower population density in an environment closer to the one that they were adapted to in Africa might actually have evolved less than Neanderthals did in Europe in 200,000 years with a higher population and an environment quite different from their ancestors.
Now, none of this is definitive. It is a toy model of evolution. Perhaps Asia was a more welcoming environment than Europe and could support a larger population as a result. Perhaps smaller individuals allowed for greater population numbers. Perhaps at the biochemical and immune system level that most of the genome is devoted to (more of our genome goes towards providing instructions to building various subparts of our bodies like proteins and tissues than to the large scale morophology of our bodies), Africa and Asia aren't so similar.
But, the notion that hominin evolution may have been slower for archaic hominins because their small populations caused them to be mutationally limited, i.e. not able to give rise to many beneficial mutations to allow those mutations to be selected positively for, could be important to making sense of the pace of evolution.
Also, if one relies on the rule of thumb that it takes 75-100 individuals to support a sustainable population of a species without intentional outside intervention, and consider that the total population of Neanderthals or Homo Erectus at far less than their peak populations when modern humans come on the scene, it becomes easier to see how modern humans could fragment these archaic hominin populations to below a tipping point and wipe them out without embarking on a full scale, Nazi-like deliberate campaign of continent-wide genocide.