Drysuits always look bigger than you think - try one on, and you’ll see.
The seals need to be tight but they do stretch to enable you to put the suit on.
You’ll need to remove the air from inside the suit by crouching down and opening the neck, the suit will then mould to the body.
Use the internal braces to adjust accordingly, the zips can be stiff when new so be sure to fully lock them into place before use. Over-the-shoulder zips always hold the material and pull the zip down rather than pull the zip outwards which can pull the zip off its track.
Take care of any sharp objects such as fingernails and jewellery which may damage them.