The Buchon was a development of Bf109 in Spain only. It was done because the shortage of DB engines and parts for repairs. The engine used was a Rolls-Royce Merlin (I think it was a 61, but dont quote me as I am talking by memory), not a "mercedes" (?)
The bulges were caused by the row of Cylinders of the Merlin, shaped in upwards-V, while DB600 series had INVERTED Vee cyilinder configuration. SO to make room for them, the cowl was adapted with the HUGE bulges. If you have seen the film "the Battle of Britain" with Christopher Plummer, the Bf109 in that film are purchased spanish Buchons.
Other side-effect of the MErlin adaptation was that the 109 cockpit was very noisy.
109Gustav series had bulges on the cowling...big ones, but not by far the ones in Buchon. As someone said, G6,G14 featured these bulges due the adoption of Mg131 in place of MG17 machineguns...
Just some notes from a Spanish 109 nut