A great many of them did come from Southern Spain. There have been several studies regarding this issue. (I am actually writing about the demographic makeup of Venezuela today for my dissertation so I have all of these books on hand.) The most famous of them was done by the late James Lockhart in his The Men of Cajamarca, which describes the demographic makeup of Pizarro's men and paved the way for biographical sketch studies of the conquest period. According to Lockhart, this is the origin makeup of the Pizarro campaign:

Extremadura - 36 men, Andalusia - 34, Old Castile - 17, New Castile - 15, Leon - 15, Biscay (Basque Spain) - 8, Navarre - 2, Aragon - 2, Greece - 2, Unknown - 37. (pg28)

In his similar work, Avellaneda Navas's Los Compañeros de Federman and later work Conquerors of the New Kingdom of Granada sketches the makeup of men coming to New Granada (Bogota) in the mid 1530s. He argues that the makeup was somewhat different since it was so late in the conquest, but overall it is not remarkably different. To help he supplies the stats for the other areas of the New World. For example, in Panama, Chile, and New Granada, Andalucians made up 34.7%, 22.5%, and 15% of the makeup, respectively. Extremadurans in Panama, Chile, and New Granada made up 21.4%, 15.4%, and 2%, respectively. For those same three locations, Old and New Castile made up at least 10% for each campaign save for New Granada where men from Old Castile made up 25% of the entire party. Overall in New Granada, 69 men came from Andalucia, 40 from Old Castile, 32 from Extremadura, 28 from New Castile, and 25 from Leon with a handful of other men from various other parts of Spain. 11 came from Portugal, 4 from France, 3 from Germany, and 2 from both Italy and the Netherlands. (pg395 Los Compañeros; pg 59 New Kingdom)

Hugh Thomas made a collection of biographical sketches for the Mexican conquest in Who's Who of the Conquistadors. In Mexico under Cortes, 148 men came from Andalucia, 69 from Extremadura, 54 from Leon, 37 from Old Castile, 22 from New Castile, 16 were Basque, and 12 from Galicia. A handful of others from the rest of Spain were with Cortes but he also brought along 10 from Portugal and 8 foreigners.

Narvaez, who unwittingly reinforced and resupplied Cortes, brought 132 men from Andalucia, 0 from Leon, 56 from Extremadura, 42 from Old Castile, 32 from New Castile, and 11 Basque men and nearly 30 from other parts of Spain. He brought an additional 11 and 17 from Portugal and other foreign areas, respectively. (pgxxi)