Read more at the linked thread and it is explained there. I just posted again at that other thread another summary post.

Large blocks are a weapon which kill decentralization due to the issue of propagation and relative orphan rates. It is a bit complex to grasp.

I proved/explained that Bitcoin Unlimited's whitepaper (written by @Peter R) is flawed. There is no equilibrium with unbounded block size and rather it is a weapon for the mining cartel to achieve full centralization. Several smart developers have peer reviewed my analysis.

But OTOH, Core also ostensibly has a sneaky agenda. <--- click and read