This makes it sound like you're treating 'size of the economy' (GDP) and 'size of the economy' (amount of money) as equal. But they're not. In the US, GDP is about $22T/year and amount of money is about $7T (so the average dollar is spent about 3 times a year). So if the government wanted to spend 10% of GDP, it would have to print $2.2T which would cause about 2.2/7 = 31% inflation. In fact the government spends 35% of GDP, so inflation would be around 100%. The value of the dollar would halve every year.
In other words, MMT is about understanding the mechanism behind inflation. If you know how inflation works then you have full control over it. You can raise and lower it at will and use this power to actually achieve policy goals.
For example, by injecting money exactly where it causes inflation you can directly cause inflation. It sounds obvious in retrospect and it explains why current central bank policies fail to create inflation.
The money never reaches the hands of people who actually spend it.
Once you reach your policy goals you can stop printing money. Meanwhile if you create money fruitlessly without achieving policy goals you end up creating more money than necessary.
The irony behind MMT is that by printing more money you end up printing less money.
Suppose you have a society with a total economy of say 10^9 dollars, and you need to allocate 10% of the to the government for it to operate.
The conventional approach is various taxes. Income taxes, sales taxes, VAT, property taxes, the list goes on and on.
The question the book asked is why not just have the government print the money it needs?
Printing 10^8 dollars would not quite do it because of the inflation that would cause, but if it printed 1.11 x 10^8 dollars then the government would have enough to buy 10% of the inflated economy.
So why not replace taxes with printing money to fund the government?
It has a couple of clear advantages. First, no more need for everyone to file tax returns, employers to withhold tax payments, and all that jazz, and no need for the government to have a giant tax collecting apparatus or a giant tax code. Second, it makes tax evasion a lot harder.
There are some downsides too. First, it would take away a large part of the usefulness of taxation as a way to change behavior. Deductions, credits, progressive rate structure--all gone. Deductions and credits could be replaced with government subsidies, but if you aren't careful you will end up recreating much of the complexity and bureaucracy that the old tax system had.
Second, the inflation would be significant enough that it wouldn't take many years before even small things involve large numbers of dollars. This could probably be addressed by going to an almost all electronic money system so you could just periodically re-scale the whole thing.
Third, inflation from printing money doesn't affect all assets equally, so this would encourage people to try to hold more of the wealth in assets that are less affected. People lean toward that anyway to deal with normal inflation, but with the current system if there is too much off that the government can use tax incentives to nudge things in the other direction.
I remember this leading to some interesting discussion in class. Even if it is not practical, thinking about it is interesting. It can get you to really think about what money really is.