When using laser radiation for ionization in ion mobility spectrometry, a large increase in the number of produced ions compared to e.g. standard radioactive sources can be achieved. This number can be so large that Coulomb broadening of the measured mobility signal can be observed. From a large enough charge density on, the Coulomb repulsion leads to effects on the motion of the ions that manifest themselves in deformations of the signal in addition to the broadening. Here we have investigated with help of finite elements method simulations the cause of this deformation and compared the calculated signals with measured ones. An important result is that the change of electric field strength when entering the drift region and its effect on the ion cloud distribution is a main factor responsible for the signal deformation.