Why Unexplained Weight Gain?
The weight increase indicates that you have eaten about 35 (kilo)calories too much per day since age 25, or about 1 % too much on average. This corresponds to about 2 lumps of sugar/day. When overweight people are questioned about their eating and are then given only the amount stated but under controlled conditions, they usually lose weight very quickly. It may be that they are not aware of how much they eat. It is not possible to become overweight without eating too much. In order to be more aware of how much is eaten it is a good idea to keep a daily food diary. More
Your wife is also somewhat plump. This may indicate that she puts more butter, cream and sugar in the food she prepares than you are aware of.
It would help you to exercise as it contributes to using calories and the normal hunger and satisfaction feelings function better. If you exercise for at least 12 minutes per day and get the pulse up to about 130 it will be easier to manage your weight problem.
Some people cannot be slim, however much they diet, as they have large numbers of small fat cells which cannot be made smaller. Those who are overweight as children usually have this condition whereas those who get overweight later in life have fewer but larger fat cells and this applies to you.
Only a new life-style for the rest of your life will solve your weight problem. Skip sugar and saturated fat.