Only allowed 100 queries with relatedto?
The reason this happens is Craft has a default limit of 100 for all ElementCriteriaModel objects (this includes assets, categories, entries, tags, and users, also relations as you encountered). 

This isn't made terribly clear in the documentation, but you can see it under the defineAttributes() method on the ElementCriteriaModel class reference page.

As you discovered, adding a .limit(null) parameter overrides that limit.