A lot of people stop eating meat and then start inhaling nuts, grains, legumes and various forms of carbs/sugars in addition to those but think that because they're not eating meat that nothing they are eating can cause weight gain.
Meat doesn't come with the omega 6 or carb load (or lectins, phytates etc) found in veggie options.
These things promote bodily inflammation and a wacky insulin/blood sugar balance not to mention gut irritation and damage---which at best leads to a lot of water weight and at worst a downward spiral of health issues and weight gain.
I, like you don't like to eat meat due to cruelty reasons. However, I realized that for me my body was made sick by veggie replacements. So, I realized that yeah, it sucks animals have to die---but every living thing takes the life/energy of another (even grass is alive) and when I die I give my body back as energy to the earth. Instead, I eat less meat and make sure its organic, grass-fed and treated well before they're killed and butchered.
Added bonus---happy meat is healthier for you.
Your other option is to partake in dairy and eggs as a replacement, though dairy can present the same carb/insulin issues.