Learn Dutch: De motor is veelbelovend, zij het een stuk groter. - The motor is promising, albeit a lot bigger.

Zij het translates to "albeit" or "be it". It's used as a concessive connector to add a restriction or contrast without needing to form a full subclause. Instead of saying hoewel hij een stuk groter is (although it is a lot bigger), you can concisely attach the contrasting trait een stuk groter directly to the statement using zij het.

Could I use hoewel instead of zij het here?

You can't just swap them directly without changing the grammar. Hoewel (although) requires a full clause with a subject and a verb at the end: ..., hoewel hij een stuk groter is. Zij het is highly useful specifically because it connects a simple phrase or adjective directly to the main clause, bypassing the need for a full subclause.

Why does een stuk mean "a lot" here, instead of "a piece"?

When een stuk (literally "a piece") is placed before a comparative adjective like groter (bigger) or beter (better), it acts as an adverb of degree meaning "a lot" or "quite a bit". Therefore, een stuk groter translates to "a lot bigger".

How is the word veelbelovend formed?

It is a compound formed from veel (much/a lot) and belovend (the present participle of the verb beloven, to promise). Together, they literally mean "much-promising," which corresponds to "promising" in English.