Terraforming small worlds seems like it would be too easy.
Consider the following: If there is 20% of the gravity, then the air will weigh 80% less and you will need 5x as much air in order to build up a certain level of pressure on the planet (assuming you dont go full gas giant and add enough air to significantly change the gravity of the planet).
This would make terraforming tiny worlds more realistic, in that it should probably be an exceedingly difficult thing to do.
So, divide the terraforming time by the surface gravity. 10 years / 20% surface gravity (0.2) = 50 years.
e: Fiddling with the math, assuming constant planet density the divide-by-surface-gravity balancing approach would result in a very nearly flat rate terraforming cost. Minding that in reality smaller planets are generally much lower density than larger planets which would probably cause them to spike massively in terraforming cost because lower density = higher radius = lower surface gravity = longer terraforming time if the suggestion is implemented.
graph (x axis is mass, y axis is terraforming time):
https://www.desmos.com/calculator/hepyghpcyhAdditionally, it might be fun if you needed to keep some number of terraformers on the planet in order to keep it terraformed. This would raise the cost in that you can't really re-use infrastructure for that purpose. I don't have a specific idea of how to balance that one.