In order to solve this issue
http://forum.nginx.org/read.php?2,255421,255438#msg-255438
two additional features would be cool:

proxy_cache_min_size

Syntax: proxy_cache_min_size number;
Default: proxy_cache_min_size 1;
Context: http, server, location

Sets the minimal size in bytes for a response to be cached.

proxy_cache_max_size

Syntax: proxy_cache_max_size number;
Default: proxy_cache_max_size —;
Context: http, server, location

Sets the maximal size in bytes for a response to be cached.