As Vint mention, Megaprims are sort of a legend in Second Life. Apparently, they were created first by Plastic Duck - an early notorious (perhaps exaggerated) griefer. An interesting interview with him was done by the Second Life Insider. The Second Life Insider also has an interesting article and history on Megaprims by Aimee Weber. An excerpt from the article:

True to his nature, Plastic Duck’s megaprims are actually the result of a wanton disregard for Linden Lab’s TOS. Exploiting the fact that prim size limitations were enforced by Second Life’s client software, Plastic simply created his own makeshift client which allowed him to log on and create prims of any size he damn well felt like. Sadly, he only felt like creating a small handful of megaprims before Linden Lab repaired the exploit by placing prim size enforcement in the server software. This ended the potential for oversized prims forever.

Since then, Plastic’s small handful of megaprims is worth their weight in gold to professional content creators. They’ve appeared as an essential part of many popular builds and are frequently used by many of the big content creation companies.

So, there you have it. The set of Megaprims we have is apparently, all we have to work with for now.

-Veyron