In practice, that's always true. But in theory, a BigInt could be bigger than G(64), as the spec places no hard-coded limits on their size.
It's unfortunate that Map and Set came before BigInts, because (unlike String and Array, whose lengths fit in a 64bit register) their internal size is unbounded, but gets converted to a Float64 by their getter
And floats lose fractional precision as the order-of-magnitude increases