MTB sizing comparing Road fit to MTB fram sizing

[cuda]

Im looking at picking up a fatbike for the winter and have come across a deal on an previous seasons model. I would normally look for a medium but all that is left is small. and I like the review as well as the front suspension on this bike.

The bike is online( boo, i know I suck but the brand is not that easy to come by in regional shops) so I cant go out and test ride.

Anyhow, here is my thinking. I dont mind having to get a longer stem or seat post to make up any fit issues. I also kind of like the idea of riding a slightly smaller frame size since these bikes are monstrous to begin with.

When I compare reach from my M Pivot Vault to the Small Cube Nutrail the difference is 7mm.
To me this seems negligible.

so the question is is it safe to compare the geo of my XC bike with a MTB and are there other specs I should be looking at?

thanks
cuda

[dddd]

It's wise to consider what stem length might be needed to achieve a good fit on a particular bike, as different headtube angles are more or less tolerant of longer or shorter stem length in terms of handling well while off of the saddle.

Too short of a stem with a steeper headtube angle makes for a twitchy feel, not always a bad thing when going slowly uphill.
But too long of a stem with a shallower/slacker headtube angle at some point makes the steering "heave" to each side as one pedals while riding off of the saddle, which requires some effort to control.

Likewise with the toptube length versus seattube angle, a shallower seattube angle steals forward reach from any given toptube length, while a steeper seattube angle pushes the toptube and headtube forward, so will necessitate a shorter stem.

So headtube angle, seattube angle and toptube length are all inextricably connected, and affect what length of stem is needed for fit and is tolerable in terms of the bike's handling.

When a frame is sized using published "stack" and "reach" dimensions, and when one wants to compare different frames, note that the reference height of the reach measurement, i.e. the "stack" dimension, affects the reach dimension itself!
So comparisons need to be made between frames of closely similar stack height, or a compensating reduction of roughly 2-3mm of reach per cm of stack height difference needs to be applied for any actual reduction of stack height.
In other words, a shorter stack height makes the frame's published reach dimension larger, even as the distance between the seattube and headtube might be the same, in which case you might end up with a slightly short-fitting bike that then perhaps needs a longer stem.

So at the end of the day, stack and reach dimensions have their own bit of complexity, just as comparing the virtual toptube lengths of any two frames also needs to consider both frame's seattube angles.