nginx 缓存文件时，不足1m安装1m算，为什么？

我在用nginx做缓存的时候，发现一个现象，nginx会实时管理缓存空间大小，比如我设置缓存max_size=100m，
nginx会实时监控当前缓存空间大小，但是问题是：当nginx缓存的文件大于1m时，nginx的监控正常工作，记录缓存文件大小，
可是nginx缓存的文件小于1m的时候，nginx监控会把新缓存的文件按照1m大小计算。求教

nginx缓存的源码：
ngx_http_file_cache_update(ngx_http_request_t *r, ngx_temp_file_t *tf)
{
off_t fs_size;
ngx_int_t rc;
ngx_file_uniq_t uniq;
ngx_file_info_t fi;
ngx_http_cache_t *c;
ngx_ext_rename_file_t ext;
ngx_http_file_cache_t *cache;

c = r->cache;

if (c->updated) {
    return;
}

ngx_log_debug0(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
               "http file cache update");

cache = c->file_cache;

c->updated = 1;
c->updating = 0;

uniq = 0;
fs_size = 0;

ngx_log_debug2(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
               "http file cache rename: \"%s\" to \"%s\"",
               tf->file.name.data, c->file.name.data);

ext.access = NGX_FILE_OWNER_ACCESS;
ext.path_access = NGX_FILE_OWNER_ACCESS;
ext.time = -1;
ext.create_path = 1;
ext.delete_file = 1;
ext.log = r->connection->log;

rc = ngx_ext_rename_file(&tf->file.name, &c->file.name, &ext);

if (rc == NGX_OK) {

    if (ngx_fd_info(tf->file.fd, &fi) == NGX_FILE_ERROR) {
        ngx_log_error(NGX_LOG_CRIT, r->connection->log, ngx_errno,
                      ngx_fd_info_n " \"%s\" failed", tf->file.name.data);

        rc = NGX_ERROR;
                            } else {
        uniq = ngx_file_uniq(&fi);
        //change by liangc
        //fs_size = (ngx_file_fs_size(&fi) + cache->bsize - 1) / cache->bsize;
        fs_size = ((&fi)->st_size + cache->bsize - 1) / cache->bsize;
    }
}

ngx_shmtx_lock(&cache->shpool->mutex);

c->node->count--;
c->node->error = 0;
c->node->uniq = uniq;
c->node->body_start = c->body_start;

    ngx_log_error(NGX_LOG_ERR, ngx_cycle->log, 0,
                   "0............................ngx_http_file_cache_update : cache->sh->size = %d ,c->node->fs_size =  %d fs_size = %d ",cache->sh->size, c->node->fs_size, fs_size);
cache->sh->size += fs_size - c->node->fs_size;
c->node->fs_size = fs_size;

if (rc == NGX_OK) {
    c->node->exists = 1;
}

c->node->updating = 0;

ngx_shmtx_unlock(&cache->shpool->mutex);

}

主要是上述代码中的：fs_size = (ngx_file_fs_size(&fi) + cache->bsize - 1) / cache->bsize;这段代码会获取当前缓存的文件 大小，
ngx_file_fs_size方法的代码：#define ngx_file_fs_size(sb) ngx_max((sb)->st_size, (sb)->st_blocks * 512)，这段代码会把换的文件大小与1m比较，去大值
，求教，有人知道这个是为什么吗

有点钻牛角啦，类似 小孩子不满一岁按一岁算，