You are (perhaps on purpose) ignoring two crucial pieces of information;
1. Frazier was shown the TSBD bag on Friday evening (at around 11.30 pm). This was the first time Frazier was asked about the bag and he would IMO have no way of knowing anything about it's later significance. This was also before the bag was ruined with silver nitrate (by Latona) and thus still had it's original color. So, while he was being polygraphed, Frazier instantly dismissed the bag shown to him as the one he saw Oswald carry.
Are you claiming that Frazier, while being polygraphed, purposely lied about the size of a bag which more than likely was insignificant to him at that time?
2. Frazier told the DPD officers that same night that the bag he had seen Oswald carry was "definitely a thin, flimsy sack like the one purchased in a dime store".
You can believe and argue all you want that Frazier and Randle underestimated the size of the bag, but how do you explain the points I have raised?
Simply by ignoring them, perhaps?
You wish.
Frazier that being threatened with being charged with being an accessory to the murder. By transporting the assassin and the rifle contained within a bag. This went on for many hours and was still going on at 11:30 pm.
Given this pressure, it is natural that Frazier would:
1. State the bag he saw with Oswald was too short to hold the rifle.
2. State that the bag he saw with Oswald bag was too flimsy to hold the rifle.
3. State that the bag presented to him, which was long enough and not too flimsy, was not the bag he saw Oswald with.
Given the pressure put on Frazier to confess, would it not be natural for Frazier to convince himself of this? If any of claims 1, 2 or 3 are true, the charges of the state against Frazier collapses.
Your airily claim that Frazier would not have known about the significance of the bag at 11:30 pm is false. He had already been questioned about it for hours and strongly urged to sign a written confession. Of course, he knew the significance of the bag presented to him at 11:30 pm.