英文解説
The first reason why a large company is better to work for than a small company (try to paraphrase the question more accurately) is that a large company has a bigger business opportunity than a small company. For example, in the market of personal computers, the large companies are dominating the market. According to a market research, it is found that about 95% of the PC market is occupied by large PC manufactures, such IBM, HP and Sony. Furthermore, the automobile industry is also dominated by large companies such as GM, Ford and Toyota. This result explains that a large company has bigger business opportunities. While a small company has an advantage in getting into a niche market, a large company can dominate the market.
解説

This example does not explain why it is better to work for large companies, which makes it look like you could not understand the question. Perhaps bigger business opportunities mean more managers, more control, or even more pressure. Be sure to relate your ideas to what the question is asking for.