At different times in history, some human populations have been consistently well-fed. They usually managed to grow large in these cases. The Vikings at their peak seem to have eaten well and grown robust. The early settlers in North America were full of admiration for the physiques of the Mohawk warriors. Hawaiian royalty made being well-fed and large a status symbol. Some of them got quite impressive.
Exactly so. Without more specimens, it's hard to really make any kind of firm inference about the true average size of Homo erectus. What we do know is that there were some reasonably sizeable members of that group, so it's not hard to imagine that the average could have tended more towards the high end of the specimens we have found. And since Homo erectus undoubtedly lived in multiple places, comprising multiple populations, I would expect a certain amount of regional variation in size, based on the relative availability of a nutritional food supply.
Indeed, there's a pretty good analogue to that kind of thing here in the modern world. For many centuries now, the Japanese diet has been heavily based on things like fish and rice, with the result that, traditionally, the Japanese have tended to be noticeably shorter than their Western counterparts, who have a diet heavier on relatively high-calorie, high-fat foods - pork, beef, eggs, milk, cheese, and so forth. But as Japanese children gradually shift to a more Western diet of burgers and pizza and the like, they are increasing in average height as well, away from the 5'6" or so of older Japanese men, and towards the 5'9" or so of Western men.