Miyata's butted frames typically use 26.8mm (approx. 1.055") seatposts. During this period, Miyata manufactured their own tubesets and chose to use a double butted seat tube, unlike most other tubing manufacturers, who used single butted seat tubes. Single butted tubes are butted (i.e. thicker) only at the bottom of the tube, where it inserts into the bottom bracket shell, because of the higher stresses at that location. Miyata's extra butt at the top of the seat tube, means it uses a smaller seat post than most single butted tubes.
The advantage of having the extra butt, in conjunction with in-house tube manufacture, meant that Miyata could control the top butt diameter, so that they only had to stock one size of seatpost, regardless of the thickness of the un-butted section of the tube. Bicycle brands who used tubing with single butted seat tubes could also stock a single seat post size, but to do so they had to ream or hone the seat tube to size of the thinnest gauge tube, an operation which added extra cost. The only other major tubing manufactuer who regulalry used double butted seat tubes was Tange.