The bigger refuellers are more important due to their greater amounts of transferrable fuel and long range. Smaller refuellers cannot be used the way refuellers are meant to be used because they themselves need a lot of fuel for their own needs. It's fine for tactical use, like a larger buddy refueller, but it's too expensive for that.
For example, the A330 carries 110T of fuel, the EC-390 has 23T. Just 1 MKI will swallow up all the transferrable fuel, whereas the A330 can fill up 4 MKIs twice and still have twice as much fuel as the EC-390. So, the bigger the jet, the more useful it is.