It is certainly worth considering, but the size of the piping is linked to its fluid carrying capacity and that is likely to be the controlling factor.
Hypothetically, a smaller pipe has a larger heat loss impact than a larger pipe.
Consider the fact that the ratio of pipe's circumference to crossectional area is 2/r; the same ratio as applied to volumes is why babies lose heat faster than adults.
Additionally, smaller diameter pipe means that maintaining the same mass flow will require higher fluid velocities, which means increased pressures, possibly, as well as potentially increased pipewall erosion problems.
Adding insulation is typically your best bet. Changing pipe size can have unintended consequence as others have pointed out. Reminds me of the guy who piped his house water at 3/4" to save pressure... had to wait (0.75 / 0.5)^2 times longer for his shower to get hot.
Main point I wanted to make is to be cautious if you are insulating small pipe... insulating can increase the heat loss! Increased surface area can more than make up for increased insulation. Limit is set by dividing k by h. {W/(m °C)} / {W/(m^2 °C)} = m. k/h = critical radius. If you are under the critical radius then you increase heat loss.