Turned and Topstitched (also referred to as "T&T") edges refer to a method where the pad is sewn together so that the "good" sides of the fabric are inside the pad, which is then turned out (though an opening left unstitched) so that the "good" sides are now showing, and the edges of the pad are finished with a stitch to help keep the pad looking neater and to give extra stability to the seam.
This provides a neater look, but involves extra layers of material at the edges of the pad, because the seam allowance is included also. So in a pad made from 3 layers of cotton, the edge would now have 6 layers of cotton (the 3 that comprise the pad, plus the seam allowance which is also 3 layers).
Topstitching also serves to compress this to make it less bulky, so provided fabric thickness is kept to a minimum, the turned & topstitched pad is not necessarily bulky. As many Overlocked/Serged pads are made with multiple layers of thicker fabrics (for example 2-3 layers of hemp fleece), sometimes a Turned & Topstitched pad can be thinner at the edges than an overlocked/serged pad, despite the extra seam allowance layers.
This extra thickness can be limited by keeping the layers of thicker fabrics, such as the core fabrics, from reaching to the edge of the pad. Either by sewing the core on to the top layer of the pad, or by sewing smaller core pieces to a full layer of fabric.