That's absolutely true, but (the standard library aside) the "syntax" -- or, rather the syntax and core semantics -- of a programming language are arbitrary axiomatic rules, while everything else is derivable from those axioms. So while it is true that a small language can lead to a not-necessarily-easy-to-learn overall programming experience, it is the only arbitrary part, and so the only part you need to memorise (or consult the documentation for when dealing with some subtlety you may have forgotten). So a smaller language reduces the need for "language lawyering" after you learn it.
Some languages (e.g. lisps) are deceptively small by relying on macros that form a "second-order" language that interacts with the "first-order" language, but Zig doesn't have that. It has only one language level, which is small and easy to memorise.
But yes, Zig is a bigger language than C, but a far smaller language than C++. What's amazing, though, is that C++ is strictly more expressive than C (i.e. there are programs that could grow exponentially faster in C than in C++, at least without the help of C macros), but Zig is as expressive as C++ (i.e. program sizes may differ by no more than a small constant factor) while being much closer to C in size, and it achieves that without the use of macros.