There is a certain binding energy that keeps a nucleus together and that does generally increase with number of nucleons (although there are exceptions and more complicated features to take into account.
However /how much/ binding energy the nucleus has when you add a nucleon will depend on what you start with. Indeed really heavy nuclei start to get /too/ heavy and adding nucleons to e.g. U235 will not increase the total binding energy as much as e.g. Ne20.
This is reflected in what I called the binding energy per nucleon. This is an average of some sort and you take the total binding energy of the nucleus and devide it by the number of nucleons in the nucleus.