I found a undocumented code about MaxLinesPerSecond in active.c.
MaxLinesPerSecond defines maximum number of new lines the agent will send.
This is documented.
But (4 * MaxLinesPerSecond) also defines maximum number of new lines in the file including the agent will not send.
This is not documented.

The code is following.
#########################################################
/* Do not flood local system if file grows too fast */
if(p_count >= (4 * MAX_LINES_PER_SECOND * active_metrics[i].refresh)) break;
#########################################################

I think it is better to be documented here. http://www.zabbix.com/documentation/1.8/manual/processes/zabbix_agentd