They take a little longer; it’s not so much the size as the time lag
to get the whole to a uniform temp. This time lag can also appear
when you have flasks invested with different investment powders, some
conduct heat faster than others (we’re not talking noticeably faster
here, but it is something to bare in mind none the less).
I’ve found in the past that if you’re in doubt giving the flask a
longer soak time doesn’t hurt things, so long as you don’t over heat
the flask (had a student take a gypsum bonded invested flask up to
1650’F once the smell of sulphur was very noticeable!), but holding
an extra 10-15mins at 1450’F before going down to the required
pouring temp most certainly won’t hurt things.
One thing to keep in mind investment casting is somewhat of a black
art, some things that work for me won’t work for you…
Cheers (and good luck), Thomas.