Slab size is part of the cost of granite countertops. Basically granite counters or just any other stone counters have different prices regarding the colour and texture of the material. However the end client doesn't seem to notice but the size of the slab has a lot of importance when dealing with the manufacturers profit.
In Europe most commercial kitchens have a countertop width of 60-65 cm, so a multiple of 70 cm in size for a slab is a guarantee of profit. Obviously most engineered stone or better quartz producers have a 140 cm slab, if you check closely, most of them actually produce several size slabs depending on the end use of the slab.
Size matters, as a warehouse or a manufacturer, if I'm in the kitchen countertop business, I need slabs to guarantee a better profit for myself, and it certainly is an important part of the business. When engineered stone is involved, and with pricing normally close or above 80€/sqm, every cm of unused material is important to account for.
I'm certain that most manufacturers when buying raw material at the local warehouse or even blocks, they tend to take the size matter into account.