They might have enough land in this park that they are not worried about running out of space before they can fit enough people in. In that sense, space efficie

They might have enough land in this park that they are not worried about running out of space before they can fit enough people in. In that sense, space efficiency probably isn't a goal. However, in the context of social distancing, 2m is a guideline, but additional distance will reduce your risk more. You can flip the question on its head and ask, if I used larger circles (such as 3m or 4m), is there an arrangement I could use to still fit the people into the area? If so, then using this arrangement should reduce risk further than using an arrangement that can only accommodate 2m circles. In other words, even if you are below max capacity and meeting the 2m minimum, different arrangements still have different levels of risk of transmission. Of course, at low enough densities, the gain becomes negligible.