When attacking from land, any ships you send with your army will carry 500 resources per ship, as the troops are able to "walk" over to the city, they require no spaces. When attacking towns on other islands, (sea and land battles), your troops must be transported there, this means you must send them with a specified number of cargo ships for your troops. The troops occupy Cargo Space. If you send 100 ships but 20 of them are filled with soldiers, you will be able to pillage 80 boatloads worth of resources providing that the pillaged town has that amount of unprotected recources.