It occurs to me that if a company is producing stained glass windows and they also make glass - why would they outsource if it can adequately and economically produce what is required inhouse (and with the added advantage of control on the quality)? Powell was a good example of this - there are pictures of the muff production method on pages 196 and 209 of Whitefriars Glass, James Powell & Sons of London, the London Museum book. 'Muffs' are shown as the cylinder method of blowing sheet glass.
As for the size, it would also allow the possiblity for the production of specialised colourways if required. More importantly, it followed in the tradition of the size of glass used in stained glass windows with the associated necessity to use lead to join the pieces together. Indeed Powell's quarries (a type of glass panel) attracted the attention of Charles Winston, an authority on medieval stained glass who wrote Hints on Glass Painting in 1847.
"He persuaded the Powells to experiment with their traditional maff, or blown sheet, glass. The sheets in 'Winston colours' that resulted met with great enthusiasm from architects of the day." page 34 - as above.
Whilst I have been writing this, there has been another post.
The Haden, Mullett & Haden pieces could only be passed of as Gray-stan to those who have not handled the latter. They are too heavier construction to mistaken as Gray-stan (and the colour is laid in differently). There is indeed a similarity with the swirling.