Here's why:
* "Quand" means "when" in English.
* "Tu" means "you" in English.
* "Ma" means "my" in English.
* "Grande" means "big" or "tall" in English.
The phrase is missing a verb or subject, making it impossible to understand the intended meaning.
It's likely that the intended phrase is one of the following:
* "Quand tu es grande" (When you are tall)
* "Quand tu seras grande" (When you are grown up)
* "Quand tu as grandi" (When you grew up)
To understand the intended meaning, you'll need more context or a more complete phrase.