Disclaimer: I have never sold insurance of any kind, and most likely never will.
Commercial insurance, such as general liability insurance is based on a combination of two factors. The first is the insurance rate which is based on the type of business you have. The second is based on your exposure, or in other words, your projected sales during the policy year.
For example, if you walk into your insurance agent's office and tell them you are selling "widgets" and you expect sales to be $100k for the year, the agent may tell you that the base premium is going to be $790. That would be a rate of $7.90 per/$1000. If you end up have $150k in sales during the policy year, then you should understand that you will owe an additional $395. Your premium for the next year will also increase because it will be based on last year's sales. If you have been told that you owe a whole bunch more money, then one of two things happened. You either understated your potential sales, or the agent did.
Unfortunately, I have heard your exact same complaint so many times that I couldn't count them all. As an accountant, I started to make it a practice to ask my client's for copies of the insurance policies so that I could measure the difference between projected and actual sales volume long before the end of the policy year. This would give me the chance to prepare my client for the bad news you're facing right now.
The bottom line is that most business owners fail to learn everything they need to know about this issue, and many insurance agents don't take the time to either fully explain to policy when its sold, or when the policy year comes to an end. You as the business owner need to educate yourself on how these types of policies work, and that starts with the right agent.