not sure since wp doesnt do anything with tweet urls… we make no conversion whatsoever… Except we the wp content filters still run to do whatever would normally happen with post content…
its wp that is generating the iframe and something on the site (theme or plugin) is adding the inline page css style for the iframe… we do not add an iframe height like that as Ike says…
what on your site is doing that? not sure… since its in page styled, seems like some js…
but we cannot give you any css to help since those added styles will take control…
guess I would try a temp switch to the default wp theme and see if still going on… if so, try disabling other plugins and see if any better… cant tell much but you have some jarida plugin that is potentially affecting images/iframes…