Most crackers are a lot less dense than most cheeses and they tend to have less fat (protein and carbs are both around 4 calories per gram, compared to 9 calories per gram of fat) so that would make sense. It's quite possible to make a cheese slice that's a whole ounce: but it's still a big slice in my book. Which is just something you'll develop an eye for over time. Speaking of which: did you look at the packaging? If it was presliced it probably had the calories per slice on it. Although honestly if you want to be more exact with your calorie count a food scale is your best friend. Even cheap ones are generally reliable and they take the guesswork out of your tracking so you won't have to wonder if the picture on a website is supposed to represent the exact thing described or if it's more symbolic.
Edit: if I'm sounding preachy and it's getting on your nerves please tell me. I don't eat wisdom by the spoonful and I'm not at a healthy weight! But I have seen a lot of people come and go here - on top of my own 25+ year on and off weight struggles and some things just seem to repeat for almost everyone so I try to save people time. But I get that it can be annoying.
