I have a big stomach even though I am very active and most of my meals are home cooked. When I eat my stomach expands so I am wondering if it could be an underlying problem I am unaware of.

This sounds to me like it could be bloating related to poor digestion, rather than it being linked to weight. Therefore, I would focus on addressing the digestive issues, rather than exercise and diet.

In general, to improve digestion I would recommend drinking around 1.5 litres of plain, still water every day. It is also really important to chew your food and to sit down to eat. Reducing the quantity of food you eat in the evening, in place of a little more at lunch time, may also help.

A nutritional practitioner would be able to work with you to address digestive issues, if you feel you need further support. You can find a list of practitioners in your area here: https://bant.org.uk

You could also read A.Vogel's digestion booklet for more information on how to improve digestion. This can be found here: https://www.avogel.co.uk/information-download/digestion-booklet-avi.pdf