Large firms are able to benefit from economies of scale. This is when long-run average costs fall as output increases. Risk-bearing, financial, marketing, technical, managerial, and purchasing are the main types of economies of scale.
A firm with a large output such as Sainsbury's is able to benefit more from technical economies of scale compared to a small off-license or grocery shop like Costcutter or Londis. If they all invested £10,000 to buy a self checkout machine, the one at Sainsbury's may get used 1000s if times in a week, whereas the one in Londis may get used less than 100 times in a week, so despite the cost of the technology being the same, the average cost of this technology is far lower for Sainsbury's because average cost is the total cost of something, divided by output.
A second example would be managerial economies of scale. Both firms may try to hire a manager for £50,000 but the manager at Sainsbury's would probably oversee dozens of staff at a time, wheras the manager at a small shop may oversee 3 or 4 staff at a time. If they are working with more staff, it is also likely that output is higher in that team or store, so the average cost of hiring the manager works out far lower.