Not purely market size, though it's a very important part for sure.
The other part is how likely a country is to try to enforce their laws, and what ability they will have to do so.
Even if a hypothetical US company had an equal number of customers & revenue in Chile as in the EU, if either the Chilean law being broken is one that Chile never bothers to prosecute, or if the worst thing they could do should they find out about the law breaking is to block the service at a national firewall level but not levy any punishments (say, if the US company has no staff or assets in Chile, and the crime has no possibility for extradition or other international collaboration to punish) then the company would be a lot less likely to comply than they are with GDPR. Because most US companies aren't able/willing to serve EU customers without having servers, employees, and revenue, physically in the EU; therefore the worst case for getting caught breaking GDPR is considerably more worth avoiding than if it would just be the EU blocking access to your servers.
The other part is how likely a country is to try to enforce their laws, and what ability they will have to do so.
Even if a hypothetical US company had an equal number of customers & revenue in Chile as in the EU, if either the Chilean law being broken is one that Chile never bothers to prosecute, or if the worst thing they could do should they find out about the law breaking is to block the service at a national firewall level but not levy any punishments (say, if the US company has no staff or assets in Chile, and the crime has no possibility for extradition or other international collaboration to punish) then the company would be a lot less likely to comply than they are with GDPR. Because most US companies aren't able/willing to serve EU customers without having servers, employees, and revenue, physically in the EU; therefore the worst case for getting caught breaking GDPR is considerably more worth avoiding than if it would just be the EU blocking access to your servers.