it's also ok to put 'is' in it,but the meaning will be changed somehow.
without 'is','used to' means something in the past,like i used to drink coffee.
if you have is there,then it will be just what you said:ge
you can try this:
the more you read,the more you know
=
if you read more,you will know more.
or:
if the terrain used to support a larger group,it will be harder to exploit that terrain while remaining in one place.
good luck!
used to 单独是一个短语,表示曾今,或者习惯于做什么.所以没问题.
至于你说的加上BE动词,be used to就表示被用于干嘛,也行.
而THE MORE...,THE MORE.表示越是什么,就会越是什么.
所以翻译就是：
一块土地越是曾今被用来支持更大的族群,就越难被开发而同时保持在一个地方