The size of the object is different than the amount of video memory that is used. On a mobile platform you get overdraw if you render more than about 3 time the number of pixels on the screen. This is true regardless of the actual size of the graphic. This is about how many of the pixels on the screen are being drawn and how many times per tick.
A one pixel tiled image, still uses all the pixels of the screen if you tile it across the whole thing, so while the object itself uses hardly any memory, your video memory is using a lot as it is storying the value of each pixel rendered. Overlapping this with another object increases the video overhead for rendering the screen.
So regardless of tiled vs. sprites, the video memory is based on pixels rendered, not on the size of the object being used to draw them. Two different areas of memory.
You can refer to the following threads for more info, also here is a quote on the mobile overdraw issue:
While we're on the subject of mobile limitations, it's worth mentioning some mobile devices can only draw each pixels 3 times per frame and still reach 60 FPS. In other words, if you have four window-sized images on screen, you cannot reach 60 FPS on such a device. (Note Construct 2 renders scenes from the back to the front, so four overlapping objects are all still rendered.) Perhaps surprisingly, this includes transparent pixels. The GPU processes the full rectangle of a texture regardless of the content, and a transparent pixel still uses up your drawing budget, so four window-sized transparent images would still not reach 60 FPS. Similarly with memory usage, this is a device hardware limitation and you'll have the same limitation with any framework.
The solution here is again to prefer using small images instead of large ones. A large image will eat up lots of the mobile's drawing budget in one go, whereas it's easier for it to render several spread out smaller objects. For example, a window-sized sprite to add borders round the edge of the screen is very inefficient, since the transparent pixels in the middle still use up the draw budget; using four separate sprites for each edge of the screen would be a lot faster.
You can find more about properly composing your backgrounds here:
The "Right" Way to do backgrounds in C2
And Ashley's blog post where the quote was taken from:
Remember not to waste your memory