这应该是引用自谚语：人心不足蛇吞象。这句谚语的意思是，犹如想吞下一只大象的蛇一般，人的欲望是没有止境的。

if you say i want to be a a snake that can eat up an elephant , it illustates that you have a great ambition ,on the contrary,if other people say that to you ,they want to talk you into giving up what you working on ! hope it would help

Agree with Chen the first and Wozitoya. 蛇吞象 means greedy. BUT,your question is intresting.Maybe have two way to explain your 想做吞大象的蛇 ,I think 1: If you use it to say others.It means that guy is too greedy.Negative mean 2: If you use it to say yourself.It means you want to make a big success in one time,or you have ambition.Positive mean

it means The human is very greedy

i'm not sure did you express it correctly. there is an idiom "人心不足蛇吞象" in chinese , especially in cantonees, which is an metaphor describes the great ambition or desire beyonds one's ability and impossible to reach, usually used to admonish people against greed. however, if someone said he/she wanna be a snake able to eat up an elephant(想做吞大象的蛇), that means he/she has a great ambition and strong will, wil try to reach the goal no matter how hard it is or how impossible it looks like