Final answer:
The pasta that is most similar to spaghetti but with a smaller diameter is capellini. This thin, delicate pasta pairs well with light sauces and is quite different from wider options like fettuccine, tagliatelle, and pappardelle.
Step-by-step explanation:
The type of pasta that is similar to spaghetti but has a smaller diameter is capellini. Unlike the wider options such as fettuccine, tagliatelle, and pappardelle, capellini is very thin and delicate, often used in light sauces or broths. If you are looking for a pasta that is close to spaghetti in shape but more fine, capellini is the ideal choice.