I THINK they are the same. Plain ol' sinus simiorum is a bit of a hoss (giant?) of a plant by itself with 3 foot long leaves and a 1.5 inch diameter cross section. If they are not the same, I would like to see a picture of the giant form.

Someone is selling this San under the name San. sinus-simiorum Giant. The leaves do look thicker. I already have a San. sinus-simiorum which is thiner and longer. Perhaps mine is still quite young?

Mine.

Here is a link that might be useful: San. sinus-simiorum

I better bow out of this conversation except to say that while I have never had sinus simiorum, I believe it is very possible neither of the 2 plants you have shown are this plant. I am basing my opinion on a picture and descriptive text prepared by Juan Chahinian in his book, 'The Splendid Sansevieria'.

It is most likely just what it says, a larger growing form of the species sinus-simiorum. There are new collections of various species being offered from time to time. It's not that it's a cultivar name, it's a physical description of the plant on offer and often added just to help differentiate from other forms and to help vendors sell more plants. No different than the oodles of trifasciata cultivars.
Not all plants of a species need to look alike, often making identification difficult. Seed grown plants are individuals after all and not clones like vegetatively propagated plants. The Thai growers often have quite a few different forms of each species available, which I find fun, from a collectors mindset anyways.
That looks like a nice plant. Did you end up buying it?

Mike