This depends entirely on the content and size of the messages.
An email consists of a header and the message body, the latter of which includes encoded attachments.
Modern email programs such as Office 365 will add about 30kb of data in the header, including message authentication, spam scores, sender metadata. Modern HTML email will add styling and layout markup, logos, and other information. HTML emails often are sent with a duplicate plain text version too. This can inflate the size of emails significantly.
According to Lifewire, the average size of an email file is 75kb. This equates to around 7,000 words in plain text.
If you receive many newsletters or marketing emails, these can increase the size significantly. Equally, attachments can be many MB each.
It is difficult to say how many emails 100mb is, but if you had 100 emails, each with 1mb attachments, this would quickly use up your email quota. That being said, compression will reduce the size of emails in your mailbox significantly.