You do not need a bigger tank, you can slow the flow down several ways. Some people use this method:
Others stuff filter foam where the water flows out of the filter.
pic from older post
you can also
Block the filter intake.
Women’s pantyhose have been used for years by betta keepers to block the flow of water through the intake, thus reducing the current. Take a brand new pair of women’s hose and cut out the toe. Wrap the filter intake in the pantyhose and secure in place with rubber bands. This method also helps to reduce damage to fragile betta fins.
You can also place tall decor/plants in front of the filter to slow down the current