@Njumaen
AFAIK size of packets is unrelated.
The opinion on the forum is that the data from tests Mikrotik publishes are largely inaccurate in the sense that they represent some peculiar lab/test conditions.
What has been observed is that coincidentally the value for routing, 25 firewall rules, 512 byte sized packet is usually a good approximation of the max speeds reachable in normal conditions on the field.
Have you ever bought socks? 
An usual way to check the size of socks without taking your shoes off is to see if they exactly wrap around your closed fist.