I was wondering is a protruding forehead really the sign of a more developed frontal lobe?

Not necessarily: Frontal bossing is an abnormal enlargement of the frontal bone. It can be associated with rare medical conditions including acromegaly, or with conditions such as benign enlargement of the subarachnoid spaces. In the latter condition, the size of the frontal lobes may be normal or even reduced due to the increased subarachnoid space. Further reading: http://www.ncbi.nlm.nih.gov/pubmed/1576605