"Why do you eat so many fries?"
Translation:Pourquoi manges-tu autant de frites ?
The 'de' here doesn't mean some, as in 'de la viande' or '[de + le =] du poulet' or '[de + les =] des frites'. It means 'of' and is part and parcel of the comparative; so with comparatives like 'beaucoup de' and 'autant de' you don't abbreviate. (I couldn't get it right at school so we had a whole lesson on it!)
According to this article, tant can replace autant in a negative or interrogative: http://french.about.com/library/weekly/bl-tantvsautant.htm
It is wrong. You can use "si" to replace "so" only when it is followed by an adjective: "Cette photo est si belle!" = "This picture is so beautiful!". "beaucoup de frites" is a group with a noun, so you can't use "si". You could always say "Pourquoi manges-tu beaucoup de frites?" = "Why do you eat many fries?", but if you want to say "SO many", then it is "tant".