It is less dense which is why it's floating, but I must be missing something. Why would a larger piece of something of the same density sink rather than float? That doesn't make any sense.
Regular white PVC should sink though. You sure its not CPVC? Is it an off white color? That may float, but I'm not sure.
PVC pipe and fittings come in either pressure-rated or DWV (Drain-Waste-Vent) varieties.
Pressure rated pipe and all fittings are made from solid PVC and will sink, regardless of size. DWV pipe usually has a "cellular core" of foamed plastic between layers of solid PVC (or ABS). Cellular core pipe floats, solid PVC sinks.