Walls do more than distinguish the inside of a building from the outside. They also hold up the roof and keep the temperature inside comfortable, so they have to be strong. But making them too thick and strong is wasteful.
Normally, you don't have to worry about wall thickness because it's determined by the width of the framing, which is standard. You can either use 2-by-4 studs or 2-by-6 ones (although you would only use the latter ones in special circumstances, such as when the wall supports extra weight or you need extra insulation). This sets a natural limit on average wall width inside the home, but not necessarily on the outside because exterior thickness is determined by the siding material.
In some special circumstances, such as when you're building with a non-standard material like rammed earth or heavy masonry, you might want to make exterior walls very thick. In fact, there's no code limit on exterior wall thickness. However, extra-thick walls limit light and can make a home feel like a cave. Maybe you like living in a cave, but not everyone does, so extra-thick walls may make your home harder to sell.
Average Wall Width of Interior Walls
Most interior walls are constructed with 2-by-4 framing, and each 2-by-4 has a nominal width of 3 1/2 inches. Drywall typically covers both sides, and it's usually 1/2 inch thick, which makes the wall 4 1/2 inches thick. Door jambs are typically milled to this width so the edges of the jabs come flush with the walls.
Bathroom walls are an exception because they are often covered with 5/8 inch drywall. This makes the walls surrounding the bathroom an extra 1/8 inch thick, but you can usually compensate for this by centering the jambs.
In basements and other areas that need extra insulation, it's common practice to build walls with 2-by-6 studs. If you cover both sides of the wall with standard 1/2-inch drywall, it will be 6 1/2 inches thick because each stud is actually 5 1/2 inches wide. You can find door jamb material milled to this width to accommodate 2-by-6 walls.
Pros and Cons of Thick Exterior Walls
One drawback of thick exterior walls is that they take up space, which is a problem if you have a limited plot area and want to maximize usable space. Moisture is another problem with thick walls. Studies have shown that moisture condenses inside fat walls as warm air from inside meets the exterior sheathing, which is colder. The moisture clumps insulation and rots the framing unless it has been treated with borates to prevent rot.
On the plus side, thick exterior walls provide more space for insulation, which makes the home more comfortable in extreme climates and also acts as a sound barrier. They also provide deep window sills and space for in-wall storage spaces and shelving. Some people appreciate the lighting effects afforded by deep windows, but others may find the atmosphere oppressive, so on this score, wall thickness is a matter of personal preference.
How Thick Is too Thick?
It's a safe bet that if you have to measure your wall thickness in feet, they are too thick, unless you made the walls from straw bales. As far as standard building materials are concerned, masonry walls tend to be the thickest, and the maximum recommended thickness for walls up to 70 feet high is 12 inches. Add an extra 4 inches for each additional 70 feet in height. Any thickness greater than this, no matter the material, is excessive.