How effective is an NDA in protecting small enterprises?

In lieu of an NDA, you are probably better off filing a provisional patent application - whatever you intend to present to the large company you can insert into a provisional patent application - in this manner, your product is evidenced by means of official documents from the USPTO and you have officially established your "invention" or "priority" date. Therefore, if the large company thereafter tries to patent your product, your patent application will serve as prior art to their application.
Note that a provisional patent application can never lead to a patent by itself - you need to effectively perfect your provisional patent application by filing a non-provisional patent application within one year from the filing of the provisional patent application, but the non-provisional application will receive the "invention" or "priority" date of your provisional application and therefore, as noted, will pre-date any patent application filed after your provisional patent application - the non-provisional application will then be examined and hopefully will lead to your patent.