Making me break out the pixel measurements... I hate you... :P
A frontal screenshot of Malphite from LoLKing's model viewer shows that from the tip of his thumb to the first knuckle is 22 pixels. From the ground to the flat part of the top of his head he is 207 pixels tall, from ground to the top spike on his head is 245 pixels tall, and from the ground to the top spike on his back he's 338 pixels tall. This means that the thumb part in question is between 10% of Malphites height and 6.5% of Malphites height. Again applying our ~5'6" soldier for comparison, that would make Malphite between 55' and 85' tall, hunched over as he is in his idle pose. I'm tempted to measure him like a turtle in this case, going to the top of his back rather than to the top of his head, which would make the final verdict 85' tall.
To keep things as similar as possible I'll use a clip from the 0:03 mark of the Galio champion teaser, where his back is to the camera but he's standing up straight. From the ground to the ankle is 18 pixels, from the ground to the rounded part of his head is 222 pixels, and from the ground to the top of his pointy crown is 235 pixels. That means that to the ankle is 8% to 7.7% of Galio's total height. Plugging in our ~5'6" civilians, that would make Galio between 69' and 71' tall, depending on if you count the crown as an accessory or not.
So not quite as extreme, but Malphite is still bigger.