It's not just rush tactics. As Stabbey said the larger army tend to get more map control in the beginning which has a steamroller effect of when your army is dead you now have more map control (and therefore resources) then your opponent allowing you to build larger armies in RTS mode faster than your opponent who couldn't secure as much of the map due to not being able to capture as many resource/building points. So while your 'starting' army doesn't have an effect unless you rush it does set you up in a better economic position than your opponent with a smaller army.
