Hi!
You can go lightweight by putting a cement/sand/perlite combo together.The thickness here would approximate what you would get with sand/cement.If you're using fibres,I would suggest that you could get a larger piece put together.
But,if you want to go thin,check out Quikwall and EIFS mesh.You can get VERY thin(for a saucer)and quite large,without the burden of too much weight.Look for the recipe and results from Laura and her 16 inch ball.Do a search using"quikwall"on this forum,you should get results.
If you want more,check my input about my own mix(MYMUD) and mesh.Excellent results to date.And Kate Moss thin!
Cheers from here

Lightweight concrete is real and it is light. There are many techniques, although most require specialised equipment. The ones you can make at home are mostly based on lightweight aggregates. Typically, the lighter the concrete the weaker it is. Using perlite instead of aggregate is perhaps the simplest approach, one of the lightest and also one of the weakest. For more strength you can try, pumice or slag aggregates. Use fibres or a complete embedded mesh to provide tensile strength and some flexural resistance to cracking.