I'd agree with the picture if that was the intent, however with a matte painting, the AAT could very well be substantially closer in that shot than the Spider Droids...
Don't get me wrong on that Sal, I'm really one to quickly accept the "varying sizes" on droids, so I'm not against your idea, just pointing out that the depth of the image can skew the scale by a margin. AAT's aren't big though, so I agree that could be pretty close then.