For one thing, the 19th century/early 20th century Highland pipemakers didn't make anything called "3/4 size bagpipes".
Many of their pricelists survive and they consistently called them "Half-size Pipes" or "Reel-pipes" or sometimes "Lovat Reel Pipes".
They were not half-size (the name being misleading) but rather around 7/8 size.
I don't know why people in the late 20th century began calling these things "3/4 size" pipes... perhaps they were thinking about violin sizes. As mentioned, the instruments are around 7/8 size.
Here for comparison are the instruments which from c1830-c1930 were called (L-R) "Great Highland or Military Bagpipes", "Half-size or Reel Pipes", and "Miniature Highland or Chamber pipes"
The "halfsize or reel pipe" above was made c1900 by Glen of Edinburgh. (The chanter is a modern "Lowland" chanter by Jon Swayne.)
You can see the size in comparison to the Great Highland pipes... they are quite easy to spot.
The chanters are not difficult to spot either, as they have a narrower bore and smaller fingerholes than the Great Highland chanter.