I am not a fat guy but I like fashion.
Just my personal opinion:
If your company doesn't require for formal dressing, Stripe button down shirt X Khaki X Brown Wing Tips shoes would be a good choice. Danny and Preppy.
Stripe shirt makes you look slimmer.
You may get that in Brooks Brother.
If you want to be hip during weekend, you may try some fine black T such as Supreme and a relax cut jeans and a pair of old school sneakers. That would be good for you. Don't forget a dark colour windbreaker and track jacket.