The Carbon Star is 2.6455 ounces lighter than the RS1.
RS1 spec lists 1575 grams or 55.5564 oz. or 3.4 lbs.
Star Carbon lists 1500 grams or 52.9109 or 3.3 lbs

I own a Star Cerwinske (non carbon) and an RS1 in gloss black, both size large. The non carbon Star is 1600 grams, 25 grams heavier than the RS1 - that's a bit less than an ounce. I can't feel any difference in weight between the two. I don't know if the Star spec weight includes the optional stick-on spoiler piece. I reviewed both helmets in detail here on Revzilla but since I own both, for your convenience, a short summary here of how they differ:

The RS1 is "cushier" and quieter. Designed with a little more "all around" comfort in mind, it has a thicker more cushioned liner and is a bit more aerodynamic in upright riding positions. It ventilates well and due to the plusher padding, seals nicely around the neck, which seals out noise and drafts up the neck. Venting inside the helmet is very good.

The Star has a leaner, firmer liner and superb ventilation and ventilation control, which makes it noisier. Airflow through the helmet is fantastic and air does come up the back of the neck. I purchased the thicker, firm cheek pads for the Star to ensure a snug, wiggle free fit. I didn't need to do that for the RS1.

Overall - I think both helmets are fantastic! Honestly, it was the Cerwinske pattern that got me to buy the Star, but once riding with it, it does offer superior airflow and airflow control and the aerodynamics at speed are excellent.

One additional note - PHOTOCHROMIC SHIELD IS AWESOME! If you ride day to night often - I commute and do - its worth every penny. Good luck. Email with questions if you want to know more.

You should not get a size smaller as it would not fit your head. According to Bell the size is measured at the person temples. If you chose a size smaller your head will not fit.
I want to buy also larger cheek pads for my Bell Star RSD (55mm firm), the problem is to find these for this helmet. It is difficult to find the cheek pads because there are several colors, Bell does not help with the information and I was told by many vendors that Bell has mislabeled the cheek pads recently...

What I will do is probably change the foam in the cheek pads for a firmer one myself...

This is an excellent helmet and I am extremely happy with it (as much that I will buy another one).

My response to the weight question touches on this issue. I did purchase the thicker cheek pads for the Star. I also tried on the size M and though I might have managed to wear the smaller size my eye location in the opening was a bit higher than optimal and the shell and protection did not come down far enough on my forehead. I would recommend trying the smaller size if you're borderline between sizes. In may case the fit of the large was perfect on the crown of my head so the best option was to use the larger cheek pads. Overall, a stellar helmet!

I followed the fit guide provided and result was a correct fit. I found the fit to be medium oval and a tad looser than my Bell RS1, but a change in cheek pads resulted in a perfect fit. I'd say follow the measurement guide and if the fit isn't as snug as you'd like front to back, call Revzilla and swap for the next smaller size. If front to back is good but you'd like it snugger across the cheeks (my situation), order the 50MM firm cheek pads. RevZilla is awesome with questions on fitting and easy about returns to get you a perfect fit so if it isn't right just give them a ring and they'll handle it.

The Helmet has a medium oval head shape. Part of the problems with these helmets, Bell as a whole, is that the cheek pads compress so much that when you buy it as a medium new, the pads compress and make it fit like a large. However, the helmets are so nice and reasonably priced its orth it to buy bigger cheek pads when yours get too compressed. Throwing them in the washer and dryer seems to bring them back to life somewhat too.

Frankly, I can't imagine getting a hot spot on the top of this helmet. The airflow is massive, including awesome flow through the two top vents. Worth noting - as good as the airflow is - and it really is stupendous - the flow control is also great - and when you close the vents, it isn't leaky. Top marks for flow and air management.

My head is 58cm. I wear a medium Shoei, but I wear a Large in the Icon Domain 2.

Hope this helps.

Snell 2010 standards on the 2010 models and up. 2011 Bell introduced the RSD Speed Freak LE. Looks sicks but I love my Day of the Dead Bell Star.

I can tell for shore that the 2010 model they improve the fit of the helmet, if you have a 2009 like i do you probably when one size down like I did. Im normally a medium so i had to buy a small for my 09 Bell Star RSD (in most cases), so 2010 and up there now true sizes. Hope this help.