The mantle plume model has been invoked to explain the formation of large igneous provinces (LIP) and associated age-progressive hotspot tracks. The shape of mantle plumes should be significantly altered by physical properties of the mantle and will influence how plume theory is used to interpret observational constraints. Based on theoretical analysis and numerical modeling, we explore the parameters that control the shape of thermal plumes in a compressible mantle. A theoretical analysis shows that the ambient mantle viscosity plays a dominant role in determining the radius of thermal plumes. This analysis is verified by numerical solutions. A continuously decreasing mantle viscosity from the CMB to the lithosphere can effectively reduce the radius of both plume head and tail. A low viscosity zone between 100 and 660 km depths where viscosity decreases by a factor of 100 reduces the radius of a plume conduit by approximately a factor of 3. Such a low viscosity zone can reduce the plume head radius impinging the lithosphere from larger than 500 km to ∼200 km. When the low viscosity zone is confined to between 100 and 410 km depths, the plume head size becomes even smaller. To form large igneous provinces, a small plume head implies time-progressive volcanism from LIP center to LIP edge.