The deformation of hemispherical sessile bubbles made of ferrofluid soap under vertical uniform magnetic fields was studied using Helmholtz coils. The deformation and the shape of the bubbles were monitored according to the amplitude of the magnetic field, the initial volume of the bubbles and the ferrofluid volume used to create them. The meniscus was found to bear most of the deformation, reshaping into a cylinder, with the remainder of the bubble forming a spherical cap, mainly adapting to the meniscus transformation. The growth of the meniscus height was rationalised using a simple model. More precisely, the meniscus shape depends on the competition between capillary, gravity and magnetic effects. These three ingredients can be rewritten to highlight two characteristic lengths of the system: the capillary and the magnetic lengths. Depending on the magnetic field intensity, the shape of the meniscus is described by one of the two lengths, thus revealing the existence of two distinct regimes.