Is the vocabulary strength of a language a determinant of its speakers development level?

I am a student of linguistics (as well as literature and philosophy) and am familiar with quite a bunch of theories relevant to this, and I think YES, though development can have a broad range of different meanings here.

Before language cortex and other relevant areas in the human brains evolved, humans had processed information purely visually and sensuously. Humans have come long past that though we still have sub-language abilities which become obvious when these centers of the brains are damaged. How patients who have suffered some type of significant head injury, whether external (wounds, bullets) or internal (strokes, tumors, degenerative diseases) process information in their brains is an example of the possible sub-language experience.

Despite the existence of sub-language experience (perhaps there is also extra-language experience, but that is another thing), all of humans' day-to-day individual experience and inter-personal transactions are processed (directly or indirectly) through language. The case of sign language is slightly different, but sign language being basically language, this does not rule out the supremacy of language in human affairs.

Words are the tissues of language, and human knowledge through the ages is stored in language. Words being the tissues of language, bits of knowledge are contained in the slots of words which in turn are stored in the brain electrically. Every sense, feeling, emotion--everything that means something--has to have a corresponding word for it to be contained and processed electrically by the brain. Without being contained in a word, nothing can be processed as knowledge. An infant experiences things but the experiences remain a confusion to it until it grows up and is old enough to use language and processes and categorizes the elements of the experiences according to its vocabulary database to make sense of the experience, be it pain, pleasure, hunger, anger, everything. The richer your vocabulary is, the richer your knowledge gets.

Languages have different sizes of vocabulary banks. The smaller the vocabulary bank of a language is, the more limited the overall experience of its speakers is. The speakers of a language with small vocabulary may say their language is rich because it can express their experiences fully; however, the point is not that their language cannot express their experience; the point rather is that their small vocabulary has limited their experience. Your love for your language is one thing, but the truth of the size of experience your language's vocabulary allows you is another.

Yes, there is a mutually causative widening relation between a language and its speakers' experience. Language grows from practical living and language allows you to experience and make sense of the world. However, when we have come long past the point when our brains stopped processing information primarily non-linguistically, the language (i.e., the vocabulary) we already have enables us to experience the new, previously unexplored areas of life. Without pre-existing information/data stored in the vocabulary (words), you cannot make sense of anything new. Without pre-existing data, any experience you have is confusion.

For you to know you don't know something for sure, your brain first searches your knowledge bank and confirms that you don't have the information of that thing you don't know. This is how your existing information begins to make sense of that something as one you don't know. Once your brain has determined you don't know that thing, it decides as to what you will do about that, based on the information you already have.