When you sew a seam in doll clothes, you will notice the first obvious difference between full-sized clothes and doll clothes. Doll clothes seams are almost always one-quarter inch. Occasionally an eighth-inch seam is used in particular circumstances where even less bulk from the seam is needed. On rare occasions, a three-sixteenth-inch seam may be used.
In fact, the entire reason we usually use quarter-inch seams is to reduce bulk. You will notice that dolls’ waists are often much smaller than a human waist. In fact, the whole figure may seem out of proportion. But there is a very good reason for that! Because of fabric thickness, when the dress or skirt is on the doll, she will appear to be in proportion. The layers of cloth are much thicker in proportion on the doll than on a person. The thin waist allows for the thickness of the cloth. So once dressed, the doll should appear relatively "normal" - at least as normal as a doll will appear.
Cloth will always appear proportionally thicker on a doll than on a person. So you have to use fairly thin fabric to sew doll clothes. This also means you want to reduce bulk in the seams as much as you can.
When you sew a seam on doll clothe, you can achieve less bulk by avoiding seams that have more than one layer in them, such as in a flat felled seam. Plain seams are often the best.
And don’t use too much back-stitching to start and stop seams. A seam which is crossed at an angle by another seam doesn’t need back-stitching to keep it from unstitching. It will be fixed in place by the seam you sew over it. In other areas, simply tie a hand knot to avoid too much bulk.
While sergers are fabulous for sewing human-sized clothes, they are not very useful in sewing doll clothes. A four-thread serger just puts too much thread into the seam and makes it very bulky. Even a three-thread serger seam will be fairly thick compared to a straight seam done on a regular sewing machine. So leave your serger for other uses. One exception is to use the serger for rolled hems on certain garments.
If in doubt, make a sample! You can never go wrong making samples.
When you sew a seam, you need to control ravelling. There are several ways to stop ravelling without increasing bulk. Obviously, from what I've just said, the serger is out... There are several good options however.
You can use a narrow zigzag done on your regular sewing machine. That will make a very neat finish for your seam. Or you can hand overcast. This is nearly invisible and adds no bulk at all. For certain situations, you can run a tiny bead of Fray Check along the edge. Don't use too much! And test on the fabric to be sure it will not stain after it is dry. Using a seam sealant like Fray Check or a similar product is the least bulky finish of all.