To get big? hmm well first off you should learn the concept of teaming. Very effective, very fast. Teaming allows you to do crazy stunts and risky things that you couldn’t do otherwise.(Like eating a virus and giving all the little guys to your team(+100 mass))
As for eating people, I usually target the people who don’t have teams. However, If I am teaming with someone good, We go for people who are teaming 
(baiting, virus shooting, etc)
Soo… Ya, That is how i get big
Agar is fun because there are so many ways to play it and not really any rules
Btw, Everyone should watch this video if you don’t already know how to tricksplit: https://www.youtube.com/watch?v=tn4IyGmfimE
You can also watch this video to learn how to do cool stunts: https://www.youtube.com/watch?v=Nl9qTzqe1QA