Sink rate. Bigger the lure, faster they sink (with exceptions, see below).
if the water is shallow and potentially snaggy, it's a brave person who ties on a 3.0 or a 3.5 if they are using $20 a pop lures.
But people also use smaller jigs (e.g. 1.5-2.0)early in the season when the squid are small and less likely to get hooked on a big jig, though they may still attack it. Smaller jigs still get down, just take longer.
Conversely, when the squid are big, some real pros have 'shallow' rated jigs in the larger sizes - theory is the more aggressive big squid will take on a big lure.
If it's very deep, there are 'deep' rated lures. Faster sink rate than the equivalent in the same size.
You don't mention whether you're going after them at night or in the day. My advice would be that if it's daytime, go with a more natural colour - baitfish or prawn.