Pretty sure we're singing from the same song sheet. If the OP's chain is the length the image implies on large/large then it will be fine on small/small - the new cassette is only one tooth smaller. As for capacity (assuming 50-34 chainset), 11-28 plus 50-34 capacity is well within spec at 33 and a 28t largest sprocket is an entirely 'normal' cassette to use. Even if the chainset is a triple 52-42-30 the wrapping capacity will be enough (39t). For a very hilly, long ride I am using 52-42-28 with a 12-30 and the wrap is not sufficient - but the RD soon tells me I've inadvertently (say in the dark) got into 28/13 or even 28/12 and I use that audible catalyst to prompt to change up to the middle ring pronto.
[/mini-rant] Why do the manufacturers produce these cassettes with an 11t cassette? With a 12t you still have a 110" gear length with a 50t chainring. Unless racing/time trialling why would one need a longer top gear? At 90 rpm cadence,
50/11 gives you 52kph;
50/12 gives you 48kph. And furious pedaling downhill will get one another 22% (at 110 cadence, if desired). I can only think that Strava may be a part culprit here. The only reasonable requirement for an 11t is with use on a 1 x 11 or 1 x 10 using a 44t chainring (say). But YMMV