What's the lore...
 

...behind the 2 different styles of wizard spires?

budget cuts

The big, over-arching ones were founded by the Combine Empire -- you can find their more detailed ruins on Kunark and Antonica. From there, I can only guess that the small pyramid-like one were built by future wizards with handed-down magic. The architecture style of the pyramid-spires is similar to the Lost Valley in dreadlands, so it may also just be the Combine Empire in a less-elegant building style.

different wizards made them you see.

the smaller spires were made by the wizards with little concern for their size while the largest of spires are the product of smallpenisyndrome.

(I have no idea)

also noted that there is only one set of spires per continent