The airline announced it will retrofit more than 150 of its planes with skinnier seats.
United says the change will allow space for six additional seats in the cabin.
The seat size will be reduced from 18 inches wide down to 17 inches. Many customers think that is just too close for comfort.
"It's already too tight," said airline passenger Hazel Smith. "You can't move, you have back aches, you already have no leg room, so I can't imagine it smaller."
United says the new slim line seats won't affect leg room.
The new change takes effect sometime next year.
The airline also plans to equip its entire fleet with Wi-Fi capabilities by 2015.