what about scrambled eggs vs oats/pasta? Eggs usually make me go on for several hours whereas pasta makes me hungry and tired after one hour

Wholefoods are key and pasta is not a wholefood. If we eat natural food sources (and listen to our body's needs), as has evolved to work with our GI tract and body systems, typically things will balance out ok.

I think it's the amout of food they consume what makes them tired, not the type of it

I am speaking from my own experience and that of every other human I have known, relative to this topic - eating meat results in sluggishness since the body has to work hard to digest it. Sure, the amount eaten is relevant, but the density of the matter applies in all cases. The GI tract of a lion is MUCH shorter than of a herbivore, which means that the herbivore can potentially hold a lot of weight of material for longer, yet their muscles continue to have access to an abundance of energy.