"There is quite a lot of sugar, isn't there?"
I believe that this is because 多い (ōi) is an い adjective. I am not an expert, but I guess you could use the construction “...たくさんある/あります”. The difference between 多い and たくさん is that the latter is an adverb! So be careful.
NOTA BENE: 多い is a special exception where ADJ + NOUN does not work. If you were to put it before the noun, make sure to change it to 多く and add の.
This is how I think it works. が introduces sugar to be the new topic of conversation. If は was used, then sugar would have already been the known topic of conversation.
In this instance for example, they could have been talking about baking a cake until the speaker brings to attention the amount of sugar, using が to put focus on sugar. Like everyone says, it really is a matter of context. It just depends on what they were talking about beforehand that determines when は and が are used.
I'm no expert but I hope this helps shed some light on it.