The growth of 4% for 25 years would nominally signify a 100% increase and you might think that the economy has double its size. But you must take into account that’s this is a compound growth then the economy would reach the double of its size before 25 years.
Think that he initial size of the economy is 10 and it grows 4% then an annual growth will be 10,4 now the compound grow is adding up 0,4 to the initial size of 10. Then you recalculate a growth of 4% for the second year this means 10.816 grow.
If you notice the extra 0.016 increase for the second year is the effect of calculating the 4% increase based on the previous size 10 plus 0.4.
