Interesting -and timely - article in New Scientist at the moment concerning the supervolcano Toba that exploded about 75k years ago.
What is relevant is that they give the amounts of magma ejected by more recent eruptions (all values in cubic km)
Mount St Helens (1980) 0.5
Mount Pinatubu (1991) 5
Krakatoa (1883) 12
(as an aside Toba was 2500 !!)
There are of course big differences - the eruptions above were explosive and dumped all their magma into the atmosphere in a relatively short period of time rather than a continual stream - but it gives an idea of scale.