There are reasons that it's more advantageous to buy bigger apartments.

Simply stated, consider the economies of scale.  The more units that you have under one roof, the less your maintenance repairs will be.  For instance, if you own 3 houses, on top of the those houses are 3 roofs.  Sometimes in the future, those roofs will need to be replaced.

If you have 9 units under one roof, you will only have to replace one roof sometime in the future.

Another reason:  If you own 3 houses and a tenant moves out, then you've lost 1/3 of your income.  If a tenant moves out of your 9 unit building, you've only lost 1/9th of your income.  It's easier to pay the mortgage and your profits aren't affected as much as you own larger and larger buildings.

Of course there are advantages to purchasing smaller units (1-4).  It is a lot easier to get financing.  The banks will give you what they call conventional financing.  If the building has less than four units, it is not considered a commercial building.  Therefore you will be able to obtain financing at a lower interest rate, for a longer amortization period and with a lower down payment.

However, if you can afford to buy bigger units, there is more room to increase revenue and reduce expenses, resulting in higher net operating income.  When the net operating income increases, the value of the property increases drastically.  For example, if the monthly net income income increases by $1500, assuming the expense is constant, then at 4% cap rate, the value of the property just increased by $450,000!!    This is how you increase your wealth and keep on building more and more.  In addition, if the property has multiple units, you can afford to hire an onsite manager to help you to be the eyes and ears for you.  Minor repairs, maintenance and administrative work can be done by the onsite manager.  This takes away the headaches of going back and forth to deal with minor issues.  You can also having to deal directly with the tenants.  

If you have small units that you would like to exchange for an upleg, please obtain consultation from an expert in commercial real estate.