All I heard was that the block size was not increased and this led to the creation of Bitcoin Cash.
Block size was increased but by using a soft fork instead of a hard fork. Bcash is also like a lot of other copycat coins that were created and its creation was not because of the fork or lack of it.
It was a fork but it is an altcoin (shit coin) while bitcoin remain the bitcoin.
Keep in mind that bcash is not a shitcoin because it was a hard fork, neither was it because of Roger Ver, Jihan Wu, etc. Instead Bcash is a shitcoin because it created and change and enforced it without reaching majority support. In Bitcoin when we want to change something (soft or hard fork) we first have to reach a threshold of supporters (almost always +90%) before we can move ahead with that change/fork. If we do anything else (like forking with 10% support) we would be creating an altcoin which would be viewed as a shitcoin.
But I am thinking recently that if the block size is increased, more transactions can be processed and the mempool will not be congested.
It depends on why the mempool was congested.
If it is because adoption has grown and more people are using bitcoin, then your assumption is correct. By increasing "capacity" the congestion would go away.
But if it is congested because of a spam attack (like this days with Ordinals scam) then increasing the capacity will only make things worse because it would make it cheaper and easier for the attackers to fill the blocks and create another congestion.
Important thing to remember is that
before the Ordinals attack we didn't have any severe and lasting congestion.
If the block size can be in a way that 1 sat/vbyte transactions can all be processed in the next block, will this not be good?
Yes it would be. And I strongly disagree with those who say congestion should exist to help miners due to block subsidy going down. Because even after the upcoming halving the miners would still be paid about $135k per block they find at the current price. This is enough incentive so that they don't need to rely on fees.
If the block size is increased, what is its disadvantage to bitcoin network and miners?
The real questions are: by how much and when?
We should always keep increasing the capacity, in my opinion. The adoption is increasing and people need to be able to use Bitcoin (BTW more capacity means more txs and more fees which addresses the concern regarding block subsidy) and there needs to be enough block space for that "additional adoption".
But it shouldn't be that big to push us toward centralization where less and less number of people run full nodes.