Much larger old_heap than heap

Hello,

I am trying to understand why the total_heap_size of a few processes
is so much higher than heap_size. As can be seen in the following
snippet, the old_heap is responsible for the discrepancy:

> erlang:process_info(QPid, garbage_collection_info).  
{garbage_collection_info,[{old_heap_block_size,1439468},
                          {heap_block_size,196650},
                          {mbuf_size,289},
                          {recent_size,11674},
                          {stack_size,35},
                          {old_heap_size,940791},
                          {heap_size,86028},
                          {bin_vheap_size,36483},
                          {bin_vheap_block_size,46422},
                          {bin_old_vheap_size,34148},
                          {bin_old_vheap_block_size,46422}]}

Why?

How can I reduce this? Garbage collecting does nothing.

Cheers,

-- 
Loïc Hoguin