Case-in-point: MapCrafter. MapCrafter is an awesome program for rendering full maps from Minecraft world files. It is written in C++ and compiles to fairly efficient native code. I use it to generate maps of the ~300 MB world on my Minecraft server, and it is very resource intesive. I allow it 2 cores of my Q6600 which it pegs at 100% for about an hour to generate a detailed map from a single perspective. Each thread takes ~2 GB of RAM, so in my case it consistently uses ~4 GB of RAM.
Yeah, I've had to give some map viewers 8GB+ of RAM just to show the entire world, so the main client using only around 400MB to load the surrounding area seems about right.