If the original image are JPEG files, you could use pdfimages option -j. From man pdfimages:
-j Normally, all images are written as PBM (for monochrome images)
or PPM (for non-monochrome images) files. With this option,
images in DCT format are saved as JPEG files. All non-DCT
images are saved in PBM/PPM format as usual.
I am not sure how to control the way convert stores the images in the PDF file, but you can use -quality and -resize to alter the compression quality.
By calling convert in one of the following ways
TMPDIR=/home/tim/tmp convert ...
MAGICK_TMPDIR=/home/tim/tmp convert ...
you can have convert use /home/tim/tmp as the temporary directory and circumvent the space problems. (Which probably has no influence on resulting file size).
What exactly is the difference between the man macro set and the mdoc macro sets?
man is minimal and simplistic, having only bare-bones commands to define section headings, paragraphs, and subheadings. The package includes some basic formatting commands like .B, .BI, and not much else.
mdoc, on the other hand, is a full-featured DSL of its own, with dedicated markup commands for common manpage elements like switches, option and parameter lists, function types and return values, flags, command synopses, and virtually everything you'd expect a manpage markup language to have. See mdoc(7) for a complete reference.
Is mdoc a superset of the man macros?
No. mdoc and man are completely unrelated packages, sharing only a common ancestry and the same goal of marking up computer manual-pages.
What I have noticed is that both mdoc and plain man share some macros
They don't share anything. It's possible that what you've seen are native Roff commands, which is the underlying typesetting language in which the mdoc and man macros are written in. These tend to be used heavily in documents which use man, but less so in those which use mdoc.
You can easily distinguish native Roff commands from macros by the way standard Roff packages capitalise their macro names. For man, each macro's name is uppercased, like .PP, .TH, .SH, etc. For mdoc, only the first letter is uppercased: .Pp, .Dt, .Sh, etc.
The Groff manpage for mdoc states that the plain man macro's are the predecessor to mdoc but does not say if mdoc supersedes man.
groff_mdoc(7)
A complete reference for writing UNIX manual pages with the -mdoc macro package; a content-based and domain-based formatting package for GNU troff(1). Its predecessor, the -man(7) package, addressed page layout leaving the manipulation of fonts and other typesetting details to the individual author.
I don't think "predecessor" here was intended to mean anything more than "something similar which was being used before". :-)
Which is more portable across POSIX systems and various implementations of troff other than Groff (like Heirloom Troff)?
The mdoc macros have been well-supported for a long time now, having first debuted in 4.4BSD and shipped with every Groff install by default. Both packages can be considered equally portable, unless you're preparing documents for antique terminals from the mid-70s. :)
Is the difference some BSD vs SYSV and/or GNU thing?
You're on the right track. Modern BSD systems use a program called mandoc to format and display manpages, which is a clean reimplementation of mdoc using natively-compiled C. It only supports a subset of the Roff language — enough so that legacy manpages display correctly.
Mandoc isn't a true typesetting program the way Groff and Heirloom are. Its focus is specifically on computer manuals: and only documents authored with mdoc are guaranteed to render consistently and correctly on BSD-derived platforms. More detailed criticisms of man can be found in man(7).
and does that make using plain man legacy?
Kind of. Unless you're an experienced manpage author with a sound understanding of Roff grammar and the pipeline's mechanics, you really shouldn't be using anything other than mdoc for authoring your manual-pages. Veteran troff users may find mdoc to be needlessly verbose or restrictive, finding man to be lighter and less intrusive. However, these authors are experienced enough to know damn well what they're doing ― so unless you're a grizzled veteran, just stick to using mdoc.
Further info
If you're interested in learning more about Roff history, and this answer wasn't FMTEYEWTK already, here're some links to authoritative learning material:
Best Answer
The following convoluted way works:
Via the [Groff] latin2 polish special characters thread on lists.gnu.org.