MRI has a pretty unique value proposition for the brain, as it can detect and discern things that other modalities (CT, US) simply cannot. Even a poor quality MRI (essentially what this is) provides more information within the brain than a high resolution CT. Many of the brain pathologies are also very time-sensitive. So a good place to start.
Elsewhere in the body, MRI is less useful, and isn't really a workhorse as it is with neuroimaging. (Except maybe joint imaging.) Most emergent problems are detectable via those other cheaper, faster, & less onerous technologies.
Your guess about size is also likely correct; the positioning of the antenna/coils and relation to the magnet are very important (especially with a weak magnet) and much more challenging/variable if you're thinking about the range of abdomen sizes (for example) versus head sizes.
Elsewhere in the body, MRI is less useful, and isn't really a workhorse as it is with neuroimaging. (Except maybe joint imaging.) Most emergent problems are detectable via those other cheaper, faster, & less onerous technologies.
Your guess about size is also likely correct; the positioning of the antenna/coils and relation to the magnet are very important (especially with a weak magnet) and much more challenging/variable if you're thinking about the range of abdomen sizes (for example) versus head sizes.