Some of the largest volcanoes in the solar system are on mars. The most likely explanation is that mars

Answer: Mars has lower gravity and lower magma buoyancy

Explanation: The gravity in mars is low. Low gravity affect the occurrence of volcanic eruptions. The buoyancy of the magma in Mars is lower and the depth of the magma chamber deeper (Buoyancy is the difference in density between the surrounding crust and the magma ascending for eruption). Olympus Mons which is larger and wider than mount Everest was formed as a result of overcoming of the low gravity and buoyancy by the magma to release enormous volume of magma to the surface.

Another reason why the volcanoes on Mars are so large is because the crust on Mars is static unlike Earth.