Large corporations have the capital to keep themselves free of rules and regulations either through regulatory capture or just being able to absorb any "punishment" that might be directed towards them when not following rules that smaller businesses, not having the ability ability to exercise regulatory capture or absorb the punishments for violating the rules, have to follow. So the larger a business is the more efficiently it can evade or break the rules.

If the only competitive pressure is to generate more profits, then the only reason to merge with another larger corporation/business in the same field is just to show growth. When the cell phone companies merge, do the prices to the customers go down? When large lending institutions merge, do the loan rates drop for those seeking loans? Do the interest rates for CD' or deposit accounts increase? The trend that I'm aware of is, no. The quality of services/goods stays the same or decreases. I'm pretty confident that my guess is correct, when businesses merge it is usually not to provide "better" to employees or customers but to buy the profits of some other business.

I'm siding with the other comments that if something needs to become a monopoly to capture more efficiency to function, it shouldn't be a private business and instead be a utility.