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.
Do MRIs have any value in replacing physical examinations (prostate, colonoscopy) with diagnostic imaging? My understanding is that assay testing is getting better, but still no replacement for the physical procedures I mentioned.
For measuring spread of disease like prostate cancer imaging can be really helpful, see: PSMA PET-CT. I could imagine a world where imaging is really cheap and agents were safer, in which case we might opt to use imaging instead of physical exams, as they would offer way earlier detection. But for now the additional radiation (not present in MR), cost, risk, time, etc makes it a no go for standard of care screening.
Imaging in general has an important role in screening for certain diseases, and in some cases is already more important than physical examination. This mainly depends on how "well" physical examinations can detect an illness, the characteristics of a particular imaging study (cost, radiation exposure, etc), and the characteristics of a disease (prevalence in a population, potential benefit of early detection/treatment). So in short, for MRI to replace physical exam and/or lab testing in screening for a particular disease, the cost needs to come down or the benefit should outweigh the cost.
Prostate cancer screening in general is no longer officially recommended by the AAFP as a rule [1], but if a patient decides to opt-in to screening, the "assay" (PSA) is preferred over a digital rectal examination, which has been shown to be increasingly useless in the detection of prostate cancer. MRI doesn't currently have a role in prostate cancer screening, but Prostate MRI has an increasingly critical role in diagnosis.
CT colonography is currently a viable option for colon cancer screening instead of colonoscopy, though colonoscopy is still considered the gold standard.
Chest CT is currently recommended to screen for lung cancer in patient's that meet certain criteria (age, smoking history).
Mammography is obviously the workhorse of screening for breast cancer, rather than physical examination. But MRI is recommended as a screening mechanism for certain patient populations with very high risk of breast cancer (e.g. certain BRCA mutations). The small population, high prevalence, and benefit of early detection combine to make this a viable option.
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.