Reduce memory overhead caused by chunk allocation

The recent overload of operator new caused a decrease in usable memory (at least with the dmc runtime) because the C runtime overhead causes an allocation larger than 64kB, probably rounding up to the 128kB. As the remaining space in the chunk is less than 64kB, it cannot be used by another chunk.

This patch allocates a little less than 64kB per chunk to avoid larger allocations due to C runtime overhead