1) The number of restaurant in the city is disproportionate/ not proportional to the population size.
2) The number of restaurant are higher than the number of customers and it causes the customer to scatter/cause the customer to be segmented with each restaurant has less customer.
* Are the above sentences written correctly? Please advise.
2 is messy, overly wordy and not natural at all. Neither "scatter" nor "to be segmented" work here (the latter doesn't really work anywhere).
I would go with something simple like:
- There are more restaurants than necessary given the size of the population.
- The city has more restaurants than necessary.
- The city has too many restaurants and they suffer from a lack of customers.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.