This situation is pretty normal. In windows the oninit process contains all memory from the engine like:
- Buffers (data caching)
- Session info
- Locking table
- Sort pools etc.
All of this is configured in your onconfig file which you can find in the etc directory of your informix dir. You can check the onconfig.servername file with an editor. Here you will see the number of buffers (in 4KB pages), shmvirtsize and shmadd. These parameters will influence the size of the memory a lot.