Is there such thing as a rootstock that instead of limiting a scion's size (a dwarfing rootstock), instead increases the scion's size and vigor (a "giganting" or "maximizing" rootstock)? For example if a standard apple tree grows to 30 feet, but on a dwarfing rootstock grows to 10 feet, is there such thing as a rootstock that could make it 50 feet? Is this theoretically possible (not just for fruit trees, but for any trees)? For example what if I grafted a normally short shrubby juniper on a taller one (like Eastern Redcedar)?
So far as I am aware, no, generally speaking, there are no rootstocks which will make a fruit tree much bigger than the size it would achieve growing on its own roots. It is true, though, that some root stocks used for roses may be used to give more vigour, and 'vigour' may also mean a larger plant than the grafted rose would be growing on its own roots, though 'vigour' more usually means a stronger growing, possibly more disease resistant and more vigorous plant with more blooms than would be possible otherwise, rather than being a much larger plant overall.
Generally, a fruit tree either grows on its own roots, or it is grafted, and one of the reasons for grafting apple trees is to ensure that the tree is a particular variety, since apples do not come true when grown from seed. Growers may also use the grafting technique as an opportunity to create a smaller tree for smaller gardens, and/or to increase disease resistance, but not generally to achieve a much larger tree than the grafted part would normally reach. One of the reasons for that is difficulty in harvesting - there's not much point in having a 50 foot apple tree if you can have one that's 25-30 feet because all those apples have to be harvested, and that's obviously much harder if the tree is very tall.
Further information on rootstocks for fruit trees here https://www.rhs.org.uk/advice/profile?pid=359, though its a UK link. More broadly, see here http://www.hardyfruittrees.ca/tutorials/2014/01/16/why-are-fruit-trees-grafted
On the other hand, with cactus, most rootstocks are "giganting" rootstocks. There are generally two purposes for grafting cactus; growing a plant prone to losing it's roots or making the plant growth explode. Most "show" cactus have spent some time on a graft.
The pictured plant is nothing like normal growth. I don't think I've ever heard of using a dwarfing rootstock on cactus. They are generally dwarfy enough.