The obvious difference is that a human on a bicycle is moving on the order of 10 times less mass than a human in a car. Like my sibling post notes, it could be different for, e.g, a motorcycle, but a cyclist also produces about 0% of the SOx, NOx, or particulate emmissions of those vehicles, whichever is a somewhat different conversation.
Well those particulates and gases are still produced in the production and transport of food. If you eat more food then more will have to be grown and transported.
Some of that food could be a carbon sink, depending on how it's grown, how it's transported and where it ends up. With the later it might even get a second life generating electricity.
