Both prints are classic examples of busy print and both create vertical and horizontal lines at the same time. So they are almost always somewhat widening.
So it goes like that: the big scale check/plaid is more widening than small, high contrast is more widening than low contrast, light/warm or very bright prevailing color(s) are more widening than dark/cool/low saturation.
Blue swatch is actually a little bit bigger than the two others.
Plaids which have a lot of clashing colors are more widening than plaids containing shades of the same color or colors that are good match and neutrals.
With plaid you can also have some effects which are more typical for stripes because contrast between two squares in a row could be less than between two squares in a column or vice versa.
There is also one thing that could significantly mitigate widening effect: rotating print 45°. Then we will have diagonals instead of vertical and horizontal lines and with right color scheme the item could be in fact slimming.
Compare this two skirts in similar colorway.
Checks of average to large scale and plaid are also (like stripes) prone to distortion effect, when garment stretches to fit curvy body. This effect is very visible.
It also usually makes darts and seams very visible and so they always draw attention to themselves. E. g. side bust darts get very visible because of that, so no decent designer does that, darts usually moved to the waist, sometimes they could be masked by lapels and pockets.
Yokes can make you thighs look wider. So if you are V shaped and you want to balance you shoulders and chest then bright high contrast skirt with yoke is something to look for.
And I really like what Vivienne Westwood does with tartan playing with its direction.
There is one more thing that is very important about this prints, but not really connected to optical illusions: they always create certain associations. Stripes are more or less neutral (of cause there are sailor and seaside holiday theme in them, but it’s not that strong), but plaid and check are laden with associations.
Plaid: Scotland and Ireland, cowboys, working class, school uniforms. So if you want elegance in plaid then cut should be really creative otherwise item will look very casual (nothing wrong with that under right circumstances) or it can make you look like you are trying to cosplay a teenager (and if it’s not intentional and very well done with right doze of irony it looks awful).
Check. It depends on what check. It can be Prince of Wales, or it can be school uniform again.
There is one more interesting effect. Small scale check, especially Prince of Wales check has somewhat smoothing effect on people who have a lot of texture (curly hair, uneven skin etc), but it looks a little odd on people who are ‘smooth’ (strait hair or or hair in accurate large scale waves, smooth skin).
The reason for that is that it creates effect of raw texture, so in comparison skin looks more even.