As a native English speaker, I would never say "her wealth is big". But, it is simple English, and most people would understand what you mean to say.
Instead I would use variations of "immense fortune".
- Her fortune is immense.
- She has an immense fortune.
- Her father left her an immense fortune.
- Despite his immense fortune, he is not happy.
I would also say:
- Despite his immense wealth, he is not happy.
The term "huge wealth" is used, though where I live we wouldn't say "her wealth is huge".
Some examples about the wealth gap (wealth inequality):
- China has a huge wealth gap.
- the causes and consequences of this huge wealth gap
- In the USA the already huge wealth gap between whites and non-whites is growing.
- There is a huge wealth gap between the poorest families and the richest.
Some examples about amassing, generating, creating and gaining huge wealth:
- Australian Mining Boom Generates Huge Wealth
- How to Create Huge Wealth With Little Risk
- Carlos Slim is Mexican, and he was able to amass huge wealth through a monopolistic control of telecommunications in the country.
- Relatives of China's top leaders often trade on their political connections to amass huge wealth.
- ... young Russian billionaires, between 40 and 50 years old, have gained huge wealth.
- Through their travels, raids, and conquests, the Vikings gained huge wealth.
You can also use the term in non-financial contexts. Two examples are wealth of knowledge and wealth of data:
- In almost all business sectors, there is a huge wealth of knowledge trapped within people.
- The teaching was of the highest level. Lecturers and tutors obviously possessed a huge wealth of knowledge within their particular field.
- The spacecraft lander's batteries are expected to last just 64 hours – that should be enough for scientists to gather a huge wealth of data.
- Facebook will now be able to tap into a huge wealth of data about real-time social interactions (when Facebook bought WhatsApp)