Topfeeding is hitting someone that is significantly higher in NW than you are. It has nothing to do about bouncing retals. Anyone can hit anyone in this game especially in express with it's high turns rate (this is why AndrewMose can come in 2 days after with 360(360) turns on hand as a republic and feel confident he will retal countries that are currently in the top ten).

The beauty about topfeeding is a tyranny, demo, or dictator with high weapons and military strat sitting on 10K acres can topfeed a country sitting on 20K acreas for 2.5K acres with ghost acres, that can be a 3.3K-4K gain in land with a single hit. After playing your turns/buying defence and you are in range or higher in NW and get retalled, they get 1.5KA, if they break you.