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AIUI there's not really anything to miniaturise beyond the rest of the pacemaker (i.e. ignoring how it's powered) - there's no 'control' or addition of material as in a big nuclear power station, it's 'just' a decaying radioactive material -> heat -> electricity (the inverse of Peltier effect heating, and presumably just as inefficient (~30%?)).

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Why not just draw heat from the person and convert it to electricity?

Looks like there has been research done on that topic[1].

[1]. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3146093/

You need a heat difference, not just heat, to generate electricity.

Now my sibling comment links to a paper where they say they can find heat differences in the body that are sufficient for their needs, so this is still a possibility! But it does mean you need to be somewhere with a heat gradient: the paper mentions just under the skin.

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