What causes lava eruptions (and shutoffs)?
Basaltic magma (pictured above) rises for several basic reasons: 1) High temperature, low density liquid mass that is buoyant relative to surrounding rock and 2) Low crustal overburden stresses that allow upward movement of the underlying buoyant magma.

Many magmas fail to break the surface if overburden keeps them "capped", but they can flow horizontally (ie between sedimentary bedding planes) if that's where the zone of weakness lies. Many may also simply lose their heat to surrounding rock and solidify on their upward migration. In many cases, buoyancy of rising magma may meet a layer of similarly dense material, effectively stopping it in its upward track.

Heat loss is important as magmas can stop flowing once relieved reservoir pressures slow upward flow to the point that rising magma cools faster than it can be replenished from below; as the flow trickles down, what's left just cools down, solidifies in place and flow stops.

On the other hand, a large reservoir contains both enormous buoyancy and, importantly, heat. That heat, constantly replenished by a large supply below, will prevent excessive conductive heat loss to surrounding rock and solidification of magma pathways, ensuring the path to the surface stays open. It's also worth noting that the local stress field changes as a magma body is emptied and can also work, along with heat loss, to cap flow.