We have measured the relation between sensor noise and the steepness (α = TR⋅ and β = IR⋅dRdI) of the superconducting transition for various superconducting transition edge sensors. The measurements are performed on square Mo/Cu bilayer thermistors of three sizes (125 μm, 250 μm and 400 μm), which are fabricated for X‐ray detection. We have identified the devices in two regimes: a) α>100 and b) α<100. The devices with α smaller than 100 strictly follow non‐linear bolometer and micro‐calorimeter noise theory while in most devices with α greater than 100, we see an unexplained noise contribution.