Tweak openQA asset size settings again
    
    The UI makes this easier to investigate now, and it looks like
    we could give x86_64 a bit more space, but other arches less.
    So let's tweak things to do that. This should also reduce
    overall usage on staging a bit, as it's up against its limits.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>