A 25.8 mm seatpost would mean very thick walled seat tube if the seat tube O.D. is the standard 28.6 mm.

28.6-25.8=2.8 mm. So, assuming standard 0.2 mm tolerance between the seat tube I.D. and the seatpost O.D., the seat tube wall thickness would be 2.8/2=1.4-0.1 (tolerance)=1.3 mm. 1.3 mm wall thickness in the seat tube is a lot (typical wall thickness is 0.6 mm for 27.2 mm seatposts).

Does the frame seem heavy?