Why it's so large for every business?
It's not "every" business - it's for business with premises. Business premises cost a fortune - rent, insurance, security, utilities, etc. Even the smallest shop costs thousands in property related expenses alone.
Perhaps the relief should have been proportional to ratable value, so smaller shops/offices got a bit less. But that would have taken time which they govt didn't have. It had to announce and pay support almost immediately to prevent immediate liquidations & permanent closures.
Don't forget that the £10k is taxable, so the govt will get a fair chunk of it back eventually from businesses that it helped survive.