You will find that a family of 5 makes cruising cost MUCH more. Here's an example*:
Disney Wonder May 18, 2020 7 night Alaskan cruise
4 people (2A 2K) in an inside room: $6,419 total. $1806 per adult and $1041 per kid plus $181 in tax per guest.
5 people (2A 3K) in a verandah room, the lowest category than can hold 5: $12,997 total. $4207 per adult and $1209 per kid plus $181 in tax per guest.
5 people in two separate inside rooms (1A 2K, 1A 1K): $9,169 total. $1806 for the first 2 guests in each room and $1041 for one kid, plus $181 in tax per guest.
If an inside room could have fit 5, which it cannot, you would have been able to pay $1806 per adult and $1041 per kid plus $181 in tax each = $7,639. But because no inside room can hold 5, you would be paying an additional $1,500 instead to get that second inside room, or as you can see, an additional $5,000+ to fit in one room.
*Yes it's just an example, yes opening day pricing would probably be lower -but not that much lower I bet-, yes you could also compare oceanview rooms but the same principle applies, yes pricing on the newer ships is a bit different due to the family oceanview rooms that can hold 5