Is it true that an eco only makes a decision on the application from the documents you have provided to them??

It would certainly seem that the vast majority of visa applications are decided without the need for an interview, so yes I imagine that the most applications are decided on the documentary evidence supplied in support.

ECO's do occasionally decide that an interview is required and it's not unknown for them to ask a couple of points over the phone if they feel there are concerns that can easily be addressed.

Most decisions are made on the documents provided, which may account for the fact that a lot of refusals seem harsh, but there may have been just one missing or unsatisfactory document which caused the application to be refused.

As TOG says, the ECO can interview the applicant or phone the applicant for clarification, etc. The ECOs also, on occasion, phone employers or schools/colleges, where documentation has been provided, in order to confirm the claimed employment or studies. ECOs, in certain countries, even contact Banks in order to confirm claimed savings, etc. Submitting a false/forged/fraudulent document, or making a false statement, can lead to a 10 year visa ban, so be warned.