Remote mining caps at 4000 mine equivalents/fleet. If you have more remote mines than that in a fleet, they will produce no additional minerals.
The concentration on the planet is lowered between each fleet mining, so it is a good idea to have big fleets at 4000 mine equivalents rather than several smaller fleets. That is, the same number of miners will produce fewer minerals if split into multiple fleets compared to fewer fleets as long as no fleet has too many miners.
To be clear, the Robot Midget Miner says it can mine up to 5kt of each mineral, so it is equivalent to 5 mines. The Robot Ultra Miner can mine up to 25kt of each mineral so it is equivalent to 25 mines. If you make a bunch of small hull miners with two Ultra Miners each, each ship would be equivalent to 50 mines and you would want 80 such ships in any single mining fleet.
At the middle late game when the ultra miners are available, it is cheaper to build 16 ultra miners with ultra mining pod than 80 midget miners. You can add 3 cloaks and still be cheaper.
Mineral concentration 1 planets will always produce the minimum amount for each fleet in orbit - (i.e. 40kt for a fleet with 4000 mine equivalents). 5 fleets in orbit will mine 200kt each year since the mineral concentration can't drop below 1 between each fleet mining.
5 fleets of 4000 mine equivalents each reduce mineral concentration 100 to concentration 34 in one turn. After 2 turns, it's time to move on.
The two reasons to use a small hull for your end game miner are gateability and cost. While the ultraminer hull can make the overall cost of the fleet cheaper, actually building those ships can only be done on well established worlds and then they have to be gathered and move through normal space. A midget miner hull with 2 mining robots on the other hand can be built at space docks across the empire and gated together very quickly.
Compiled from comments of velvetthroat57 & Ptolemy 
This article is still being written. Feel free to help :)