In terms of ionic size, the size of an ion is generally determined by its electron configuration and the number of electron shells. Specifically, when comparing ions with the same electron configuration, the ion with more protons will have a smaller size.
In this case, comparing H- (hydride ion) and I- (iodide ion), both ions have gained one electron. However, the hydride ion (H-) has a proton number of 1, while the iodide ion (I-) has a proton number of 53. Since the iodide ion has more protons than the hydride ion, it attracts its electrons more strongly, resulting in a smaller size for I- compared to H-.