You can mine spare GBs on an already turned on computer without spending hardly any extra power or money or bandwidth. That's the best ROI as there is nearly no cost, except perhaps a watt extra due to some extra disk activity. If you buy harddisks, the ROI is lower of course, because you have to earn to pay back the cost of the harddisk. If you also buy a computer, then you have to pay off extra power and the computer as well. So i'd say the small miners are the ones who get the most %age wise return on investment.
The only difference is time. You could argue that each extra TB set to mine cost less and less of your time to set up. Everything else seem to favor the smaller miners, who can mine on equipment they already have, and already have turned on.
My experience is that you have to set off some time tweaking things when you reach 20 to 30 TB per computer. That's more time for big miners. Also big miners have to make sure things run with a good uptime, as downtime cost a lot of burst, so you tend to spend more time making sure systems are online and working well.
If the burst price is low, and you are impatient, it might be easier just to buy burst, and help with the advancement of the coin ecosystem