What are the trade-offs between using a larger vocabulary and a smaller vocabulary in neural machine translation?

In neural machine translation, using a larger vocabulary and a smaller vocabulary each present trade-offs related to coverage, model complexity, and handling of rare words. A larger vocabulary allows the model to represent more words directly, reducing the number of out-of-vocabulary (OOV) words. OOV words are words that are not present in the vocabulary, and the model typically handles them by replacing them with a special "UNK" (unknown) token. Having fewer OOV words means the model can translate more of the input text accurately, leading to improved translation quality. However, a larger vocabulary also increases the model's complexity. The embedding layer, which maps words to vectors, becomes larger, requiring more memory and computation. The softmax layer, which predicts the output word, also becomes larger, increasing the computational cost of decoding. A smaller vocabulary, on the other hand, reduces the model's complexity and memory footprint. This can make training faster and easier and can also improve the model's ability to generalize to unseen data. However, a smaller vocabulary also means that there will be more OOV words, which can negatively impact translation quality. The model may struggle to translate sentences that contain many OOV words, and the resulting translations may be less accurate or less fluent. To mitigate the issues with OOV words in smaller vocabularies, techniques like byte-pair encoding (BPE) or WordPiece are often used. These techniques break down words into smaller subword units, allowing the model to represent rare words as combinations of more frequent subwords. For example, the word "unbelievable" might be broken down into "un", "believe", and "able". This reduces the number of OOV words and allows the model to handle rare words more effectively. Ultimately, the choice between a larger and smaller vocabulary depends on the specific task, the available resources, and the desired trade-off between accuracy and efficiency.