Heard some boffins discuss this on the radio. UK, USA and Canada, when compared with other countries, are fatter for historical and cultural reasons apparently.
We share several characteristics that are not shared with France, Spain, Italy, Korea etc.
We industrialised early with massive knock on consequences which include:
We eat much faster
We eat alone
We eat in front of the telly
We value being full up
We prefer cheap food over quality.
In summary, we've been conditioned to see food as a way of fuelling ourselves for hard physical labour in a time conscious world, even though many of us sit on our arses all day.