So ICE did not take any advantage of the available memory. In the case of ICE it is not fine tunning, it is rather a basic configuration.
You mentioned 64GB RAM machine - For single user I would put 48 GB for MainHeap, 4 for Compressed Heap, 4 for Loader Heap. It is advisable to set swappiness to e.g. 5 and to check from time to time if swapping occurs - if yes then the heap size should be diminished.
Although decompression takes time, decompressed data are cached in memory, so they can be used immediately if subsequent queries needs them (or the same query in later execution steps). The larger MainHeapSize, the more data can be cached. Also a second execution of the same query can be much faster due to caching of uncompressed data.
