I have a smaller kitchen than you (10x10) with subways along my range wall. I don't really think it makes my kitchen look wider. Remember, there are also vertical lines as well, so they seem to cancel each other out. While my grout is somewhat darker than the tile (creamy white), it isn't high contrast, so the grout doesn't draw attention to itself. My backsplash is more of a homogenous wall.

It might depend more on the color of your backsplash next to your other elements and its all-over shape and size.

If the horizontal lines are supported by other strong horizontal linear statements, yes, it can increase the sense of spaciousness. By itself, no, subway tiles won't work miracles into transforming a cozy kitchen into an expansive one. Look at other "tricks" beyond just subway tile if you want the maximum eye foolery. Such as doing lighter colors on the top cabinets, darker on the bottom, and diagonal flooring layout. Light reflective surfaces also increase the sense of space, so a higher sheen on the cabinets, walls, or backsplashes can help that as well. Minimize the number of focal points and don't try for "statements". Clean and de-cluttered always looks larger than full of tchockes and visual noise.

Mirrored backsplash if you can handle the clean-up? Or maybe backpainted glass.

Add a pencil line into the subway tile. For me it adds length.

This kitchen doesn't need length but the pencil line reinforces it.

[Beach Style Kitchen[(https://www.houzz.com/photos/beach-style-kitchen-ideas-phbr1-bp~t_709~s_2110) by Jenkintown Architects & Designers Asher Associates Architects

Without the black line I think this tub area would look smaller.

[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Minneapolis Artists & Artisans Clay Squared to Infinity

{{!gwi}}

However, I can' t imagine having a lot of stuff on the counter that's then reflected.

nosoccermom, yikes! Plus always seeing from chin to waist as you work. No thanks. :)

LOL linelle, you'd watch your waistline grow as you cook and cook and cook in your new kitchen...! Actually, maybe a mirrored backsplash would help me make healthier choices!

That mirrored one would surely make me less likely to "taste" as I go along!!!

I would need to dress up just to cook. I doubt my old tshirts and sweatpants would look to good in the reflection.

You could check to see if you had spinach in your teeth. But then we'd need a thread titled "Show me your kitchen dental floss storage!!" I think an undercounter one like the paper towel holder would be nice.

Our second bath had wallpaper with one of those horizontal borders. The hideousness aside, when I redid the bath, now 10 y.a., I went with white subways. It's not a big bath -- maybe 10 x 12 but the directional subways made it look much larger. So much so that when the original owner of the house came to see what we had done and couldn't believe we hadn't increased the size. So the answer to the OP's question is "yes," in IME.

But in small spaces, I like to keep it simple and minimize contrast in things like pencil liners or contrasting wall color. LWO gave you some very good suggestions.