Viscous magma can form steep slopes and ash can pile up in steep piles, so the slopes of strato volcanoes often approach 35 degrees. The volcanoes themselves tend not to be very big. Strato volcanoes occur all along subduction zones, most particularly around the Pacific Ocean. Shield volcanoes are composed of basalt lava, which is hot and fluid. This means that gas pressures cannot be built very high so that eruptions are not explosive.
Shield volcanoes tend to be very big.
There are a few shield volcanoes along some subduction zones but they are not the big kind. Rhyolite caldera complexes are also pretty large. Fortunately they don't erupt very often because when they do they are extremely explosive and the volumes of erupted ash are on a scale that is hard to comprehend. We have not had to live through one of these. They usually consist of a large caldera, sometimes 30-40 km across, that formed during a huge eruption.