They go by your Body Mbutt Index I think it was like this , but if you search for BMI on here you'll find the table no doubt.
The look harder and cat-b bits are purely and simply to look to see if you have underlying medical conditions caused by weight. Like, diabetes, heart issues, etc. If you have not got these, and have never presented with them then they will not look twice. Either of these two are also not a huge problem.
Cat-B means a referral to a Dr at this end in Aus where the Dr reviews the medical himself instead of just rubber stamping it - it adds about 2 weeks to your processing time (or used to) and this is the guy that will request further medical history information IF they think you've got a major problem. Cat-A still gets reviewed by a Dr but its a rubber stamp rather than an inspection of the forms.
There is a website where you can search for appeals for people failing visa's for medicals I had a look about 9mths ago all of the failures were to do with HIV or extremely serious life threatening illnesses.
This is all stuff I gleaned from researching this stuff last year when my BMI was well below those stated above, after 4 mths of Aussie Pies, Cold-Rock Ice Cream, $2 lager, I fear that I would not be so lucky again and need to get on the running machine ;-)
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