Hoosier-- there's more to it than just eating a piece of cake.. It really comes down to calories in and calories out.
1) What amount of calories is needed to maintain your current weight based on your BMR ,weight and activity level... (You can find calorie calculators that can help identify that) Let's say it's 2,700 calories a day to maintain your current weight.
2) How many pounds do you want to lose multiplied by 3,500 calories (3,500 = 1 lb)...let's say for 1 week you want to lose 2 pounds - you need to reduce 7,000 calories over the course of that week - or reduce your daily intake by a 1,000 cals for 7 days. So each day you would limit your caloric intake to 1,700 calories a day, using our example of 2700 as a base.
That one piece of cake is lets say 300 calories.. If you've eaten 1,400 that day- no damage. If you've eaten all your 1700 , you can reduce your calories over the next few days to account for the cake... no damage; If you eat your 1700 each day plus have a piece of cake -- not enough damage- you'll still lose around 1.5 pounds.. However, let's say that cake set off cravings, and left you hungry in the next hour -so you eat something "healthy" or you begin to eat a treat a day ..-- a twinkie (300) here, a Hoho (300) there, before you know it over the course of the week you have eaten your way passed your maintanence cals of 2700 and you are up .5 pound one week.. Then the next week you are "good", eating all the right stuff, but tooo much of it... There ya go - add another .5 pounds , then the next... and so on .
This is kinda simplified.. but it comes down to eat less and move more..
I guess because of what we eat, when we eat it, the amount of water we are retaining and when we weigh ourselves all contributes to that Up and down we see on the scale.. The best way I found is to average my weight loss over a period of a month.
I hope this makes sense to you.....