You actually have it wrong. Trade ports give an increasing amount of return the more of them you make unless you decrease the trade chain. (decrease the longest most efficient continuous trade route)
For a given trade length trade ports give neither increasing or diminishing returns. It increases linearly, so that if you double the amount of trade ports in a certain gravity well you will double the trade income from that planet. Increasing your trade chain is a global bonus to trade income, but is not directly determined by the number of trade ports.
As mentioned refineries give income based on the number of resource asteroids in the gravity well and every adjacent gravity well. However unlike trade ports there is a level of diminishing returns, or rather, a cut off point. A resource extractor can only have 3 refineries servicing it (look at the extractor info card to see how many refineries are servicing it). Once an extractor has 3 different refineries using it its like it doesn't exist, and any further refineries won't get additional resources.
Typically you'll want refineries on central planets with a lot of phase lanes to other planets with extractors, while trade chains are best made around the edge of the map in a linear fashion or in a curving pattern to get the longest chain possible. The vast majority of the time though trade is the better source of income.
