They changed from the old blanket shape to the mummy shape to get the insulation next to the body. The result is that your body has to heat a smaller room.
I recommend you to buy a very very cheap mummy shape sleeping bag, or even a used civil one or just a fleece fabric liner to try out, how comfortable you feel with the mummy shape. Perhaps the fleece liner would be the best idea, because you can use it later too.
You just can try it in your bed at home.
And then you know, if you really need that extra space.
Extra space means extra weight and volume, and a tighter fit gives you a better insulation.
Of course that doesn't really matter if you don't want to use it for winter camping, but then I would buy a sleeping bag that fits with the temperature I really expect.
And if you plan to stay just over one night you could even think about buying a civil down bag and the lightest existing Therm-A-Rest insulated air sleeping mat.
I guess you will not really become stronger in your age. And such light stuff would be my choice if I would be 70 years old.
The Robens Couloir downbag series have a centre zipper and should fit well together with the Snugpak Special Forces bivvy bag, because both have the central zipper even an aged person can enter it easily.
Robens Couloir Sleeping bags exist in several weights and volumes for several temperatures.
I am used to cold temperatures and not so very sensitive. I use the Snugpak Special Forces 1 sleeping bag until 0 *C with a very thin insulation mat, but then I put off only the waterproofs if I go to sleep.
I recommend to others - like Snugpak does it - the Special Forces 1 until 4*C with a good sleeping mat, like that light Therm A Rest mats, and to wear Merino wool underwaer in it.
The snugpack SF1 is relatively roomy even for me and I am far taller than you, I am 185cm tall.
It has no warmth collar because it is meant as a summer sleeping bag and offered together with the SF2 winter sleeping bag as a complete system. But you can get it solo too.
The SF1 weights only 1030g
The SF bivvy bag weights only 340g
The Therm-A-Rest NeoAir X light 344g
(That's an ultra light insulation mat in your size.)
That's all together only 1714g or
3,779 lbs for a complete sleeping kit!
And that is very compact too of course.
With a Robens Couloir down bag it would even become lighter or warmer. But you have to air it very well in the morning, the Snugpak SF1 doesn't really need it, because it works very well together with its bivvy bag.
So, the down bag would be mainly an option if you usually go for only one night or usually have dry weather and sun in the morning.
Perhaps you stay at home, if you know that it will rain the next days. Than it would be an option for you.
If you are interested in such very light equipment I can tell you about it.
The stuff lost round about 70% of weight in the last 30 years.