Why would the Corinthians be able to give more than the churches in the mountains of Greece?
Corinth was a cultural and commercial center, so more wealth was available there than in the poorest mountainous parts of Greece. Paul contrasted their prosperous setting with the poorer churches to encourage the Corinthians to give generously from their greater abundance.
Les explains that Paul is using a little psychology. He builds up the Corinthians by reminding them that Titus and the brethren are already making collections in the poorest part of mountainous Greece. When those men come into a cultural and commercial center like Corinth, there will be more wealth available than there would be up in the mountains. So the Corinthians could give more because their city was a prosperous commercial hub, not a poor mountain region.