Very interesting topic. To my memory, 大型纪录片 is a relatively new word. Before 大型纪录片, Chinese natives are familiar with numerous programmes begin with 大型. Some are shown above, and more examples can be obtained by searching with the keyword 大型综艺.
They are not always so '大型'. But why do they dare call themselves 大型? Well, I would say that's 中国特色.
When 大型纪录片 is used ... what does 大型 refer to?
- Many money are spent.
- Very long, many hours.
- Large-scale, as earlier answers suggested.
- Just an advertisement and absolutely no warranty!
Sometimes 大型 refers to #3, but most of the time it is for #4.
However, the term 小型纪录片 is usually only used by 纪录片专家 (specialists), and means 小型的纪录片 literally. If a 纪录片 calls itself a 小型纪录片, it is usually REALLY 小 - but still can be longer than some so-called 大型纪录片. For this reason, a 大型纪录片 needn't be longer than a 小型纪录片. Haha!