The lower leg IS pretty thin and the background actually looks more realistic than the mecha itself, but it's still quite well done. Pretty good design work on the mecha and good job making the background look like a real environment. Particularly like the impacts on the pavement, though, and how you've got bits of debris flying off from them. Gives a good sense of motion as well as saying "now that's gonna hurt".
Great motif and skillful as wel but since you wanted some critism ... I'd say the proportions of the mecha in the forground look a bit odd. His left leg is a bit thin towards the ankle and the weapons look a bit oversized. And well no something that's just my personal taste: the pic looks very clean and homogentic I'd prefer something dirty with lot's off small details like corrosion or some more tecs on the walls. But still a very nice mecha artwork.