If I attack a player city that has more resources than I have transports to take, I send all transports and workers I have and I do not get back the amount of res that should be carried, It is quite a bit less than max. If I keep sending to the city, I will eventually get all of it. These cities are inactive, some were very close and some over 40 miles. If the transports are enough to carry all the res, I get a full load, if there is more res than that I do not get a full load.
I have this on 2 different servers. All my techs are lvl 10.