Seat posts have to be sized according to the seat tube, not the seat lug, where there's often a bit of distortion. In a perfect world, you'd have access to a hole mike, or bore gauge like on of these that would allow measurement of the seat tube below the lug.

The fact that the ears touch seems to indicate that the post is probably too small, but you can't force a larger post into the tube. I suspect the lug is a bit distorted, or the ears bent bent from over tightening. Fit the 25.0 post and gently tighten it. When the ears touch, does the top of the slot touch also, or only the ears? If the latter, the ears are bent and the post is the right size.

The fix is to file the inside of the ears so they can close more, but you'll be limited in how much stress they can take, so you'll want to reduce the clamping force needed by using coarse lapping compound on the clamping area of the seat lug and post. When clamped under pressure the grit in the compound bites into both the post and lug making a mechanical engagement, not dependent on friction alone.