It most certainly is. It shouldn't be a surprise that there have been studies on the subject. It's been shown that "growers" gain on average an extra ~2.1 inches when erect, while "showers" only grow by an average of ~1.2 inches.
And while yes, these are averages, it's highly unlikely someone with a 3-4 inch flaccid cock will grow to 8 inches. Perhaps not impossible but, if possible, the odds are probably quite low.
Given what little we've seen in Dr. Mike's pics, like the one above on this page, he certainly looks to be on the smaller side while flaccid, which would make him more likely to go in the category of "grower." I would further hazard a guess that we are in fact looking at 3-4 inches flaccid. Based on that assumption, purely statistically speaking, it's more probable that is dick is very much average (i.e. likely somewhere between 5-6 inches when erect.)