Size is not necessarily significant unless is a random selected sample (and I'm not sure volunteers would count as random).
Let's say you have a population of 100 and there are two groups (A-80% and B- 20%). Let's say that individuals in group A are more keen to voluteer themselves. You could have a sample of 50% of the population and still not have a representative sample of their entire population (ie. 49 from A and 1 from B), so the sample size didn't matter to much in that case.
I would say that most surveys/studies are done on a voluntary basis.
Also most surveys/studies carefully say that they are not necessarily representative.
I think I would somewhat cautiously state that it isn't something that is possible to mitigate. There are simply two cohorts: people who are willing to take online polls and people who are not. It is inherently impossible to gather this kind of data on the latter.
For example, if we define that white males get the highest page in average, we could also see in the same statistics that middle aged white males who suffer from health issues and can't compete in their group in overtime get less pay compared to someone with the same health and age group but of different race and gender. Are white males then winners?