There are many differences between body-positivity and fat activism.
The former erases systemic and institutional forms of oppression that the latter experiences. Body-positivity is also like a watered-down, neoliberal version of fat-positivity that encourages self-love through consuming and engaging with capitalism.
Body-positivity also often still encourages "health" as a necessary factor for self-love/acceptance. The body-positive movement is a movement that advocates for Health At Every Size and encourages self-love and self-confidence regardless of the physicality of one’s body. While this sounds appealing, the body-positive movement decontextualizes fat bodies by suggesting that it is possible to have high self-esteem and good physical and mental health regardless of fat-phobic institutional and systemic forms of oppression. It’s very easy to tell someone to love themselves or their bodies while ignoring overarching systems of oppression and doing nothing to change those things.
Body-positivity also doesn’t really advocate for self-love for everyone. Within this ideology is a moral imperative to be healthy. The health standards upheld by body-positivity are based on Eurocentric and patriarchal values, and the fat bodies that are most capable of achieving these standards are able to do so because of various forms of privilege. For example, fat bodies that are represented in body-positive media (and are recognized as morally good and worthy of self-love despite the fact that they are fat), are generally also white, upper-class, conventionally pretty, able-bodied, and physically fit. Fat bodies that experience different forms of oppression (i.e. those of lower classes, the disabled, chronically ill, and/or of racial, gender, and sexual minorities) are often not represented in the body-positive movement.
The moral imperative to be healthy often rests on our desire to avoid death. It’s the same reason why people who believe in clean eating, veganism, or crossfit are so loud about it - they believe they have found the best way of eating, living, or moving one’s body and believe they have outsmarted those who do not have the dedication or insight necessary to live as long a life as possible. As fat has become synonymous with death, dying, and illness, it is seen as something to avoid at all costs.
We’re seeing more and more bigger bodies in the media - which is a step in the right direction - but the increasing the number of fat people in the media is not always synonymous with meaningful representation. We need to see respectful and meaningful representations of fat people and fictional characters with real storylines and complex character development.
Being fat positive is what will truly lead us to body liberation for all.