I'll start off with a quote that gives some idea of the scale of brewing in Burton. And reveals how impressive Allsopp's brewery was.
"Each day, as we continued our investigations, we were the more surprised at the size of the buildings, and the enormous amount of plant and machinery required to carry on the manufacture of ale in these mammoth Burton breweries. As we steamed into Burton Station from Derby, the object that principally attracted our attention was Messrs. Allsopps' new brewery, which is of enormous dimensions, handsome elevation, and ranks equal to any in the kingdom. The brewery and its attendant buildings, including the offices and cooperage, cover about fifty acres, and extend from Station Street to High Street, where it is connected with the old brewery and cask manufactory by the private railway lines of the firm. The company have also 140 acres of land for the business in the parishes of Shobnall, Stapenhill and elsewhere. The clock-tower house, as seen in our engraving, rears its head above all the other buildings, and contains the Engine House to be hereinafter described. It is of handsome and lofty elevation, and is quite an ornament to the brewery. On our arrival, we proceeded to inspect the establishment under the personal guidance of Mr. Joseph Stirk, the head brewer. The brewhouse and its attendant union and racking-rooms are fine specimens of brick-constructed edifices, are of handsome elevation, and have a greater capacity than those of any single brewery in England. They were erected 1859, under the direction of the engineer and building staff of the firm, and the brewery has been aptly designated by those in the trade as one of the model English breweries, not only because of its structural arrangements and adaptability to the uses of the art of brewing, but for possessing all the modern appliances, machinery, and vessels which have from time to time appeared and been invented."
"The Noted Breweries of Great Britain and Ireland, vol. 1" by Alfred Barnard, 1889, page 133.
The date at which the new brewery was built is significant: 1859. That's just when the sales of pale Ale really started to take off. Remember that Pale Ale brewing started in 1822, when Allsopp started looking for a new export market to replace Russia. But it was only in the 1840's and 1850's that Burton brewers began to expand in a big way.
There's significance, too, in Barnard being most impressed not by Bass, but by Allsopp. In the 19th century, the two breweries were equally famous and of a similar size. It's what happened in the 20th century that makes the Bass name more familiar today. Bass led one of Britain's large brewing groups. Allsopp was subsumed into someone else's, its name soon removed from the copper plate outside the head office.