It comes after many hospitality venues complained about new guidance released last week, which stipulates that guests from different households must maintain a one-metre distance from each other.
Industry figures had expected the rules to apply just between tables, which would allow for greater capacity.
Asked why the distancing rule also applies outside, as it is known that Covid-19 is less easily transmitted outdoors, Dr Smith said on Thursday that the guidance was about “gradients of risk”, and that this distance is unlikely to be shortened any time soon.
“Outdoor settings are safer than indoor settings, that’s the starting point. But outdoor settings themselves are not completely risk free,” he said.
He added: “One-metre is not as safe as two metres, whether you’re indoors or outdoors - that's the science behind the dispersal of this virus, particularly around droplet and aerosol spread.
“Aerosol is less relevant when you’re outdoors, but with droplet spread as soon as you get down to one-metre there is a theoretical risk, whether you’re indoors or outdoors.
“That’s why the guidance still says that even if you're outdoors, if you're not from the same household as others sitting at the table, there should be that kind of distancing in place to make sure that it's as safe as possible.
“This is all about the proportionate application of guidance to try to make people as safe as possible for the environment that they are in.
“It may come that over time, what we see is as we learn more about the way that the vaccine programme has an influence on transmission, that we will be able to give further advice on that, but we’re a good way off that yet, before we’re able to change any of that guidance.”
Hospitality will be allowed to reopen in Scotland from Monday, April 26, with pubs and restaurants allowed to serve food without alcohol indoors until 8pm, and food with alcohol outdoors until licencing laws allow.