A convenience sample uses whoever is readily available. It can be quick, but the selection process may leave out people whose views matter. Surveying shoppers already inside a store, for example, says little about people who decided not to visit.
A larger sample does not automatically repair that gap. If the same kind of person keeps being selected, adding more responses can make the estimate more precise for the wrong group. Begin by naming the population, then ask who had a realistic chance to be included.