It means: "Those who are late have to work much harder than the others to achieve the same goal within the time limit"
Explanation:
The idea behind it is that if you want to finish work or accomplish the same as the others in the same time limit, you must start to work at the same time as the others. If you start late, you're either have to put in much more effort or stay later at night finishing what you could have done earlier.
Building a house as a replica of a model means that the details are followed. The only difference between a model and the real building is their dimension. Other details stay the same. So if the the model has two windows, then the real tree house should have two windows too.