The Mercedes driver left Spa with his advantage trimmed to 17 points and the prospect of race winner and title rival Sebastian Vettel making further inroads at Ferrari's home Italian Grand Prix next weekend.
Hamilton started on pole position at Spa but was well beaten in the race, unable to match the Ferrari's speed on the straights.
Like Spa, Monza is a power circuit where Hamilton won last year - but that offers less comfort now.
"Every time we do bring an upgrade they bring a bigger one," the Briton said of Ferrari on Sunday evening.
"We’ve known for the last four races or so that they’ve had some things on their car that’s enabled them to be quicker on the straights and yeah, we’ve just got to work harder, I guess," he added.
Hamilton finished 11 seconds behind Vettel at Spa but the German passed him on the Kemmel straight like a hot knife through butter.
He had a chance of retaking the lead at the safety car re-start on lap five but recognised that Vettel would simply have done the same again.
"It’s just power. They’re able to deploy more, somehow, than us," he said.
"I think they seem to have it where there are straights. You’ve got Mexico, long straights, they will have the advantage there. They generally have the advantage at places like Singapore.
"The next race, with this kind of performance they have on straights, we might struggle to match them there," added the reigning champion who has in the past made big gains in the second half of the season.
That looks less likely to be the case this year, with Hamilton and Vettel now evenly matched and the battle of four-times champions finely balanced on five wins apiece with eight races remaining.
"When I look at today's race, I see many deficits," commented Mercedes boss Toto Wolff, even if his team stretched their lead in the constructors' standings thanks to the retirement of Ferrari's Kimi Raikkonen.
"There are deficits that are obvious which cause us not to perform as we expect. It's not about somebody else out-performing us, it's about us finding the clues to understand our under-performance."
Wolff said Ferrari had been quicker since Austria last month.
Vettel, beaten by Hamilton against the odds in Germany and Hungary before the August break, experienced a wobble in Saturday's rain-hit qualifying when he and Ferrari lost their composure, but he had no doubts that his team were on the rise.
"I wouldn't disagree that this year in terms of power, we are a lot closer than we were last year," said the German.
"I think last year we didn’t have a chance here ... it’s good to see that we’re making progress."
(Reporting by Alan Baldwin, editing by Nick Mulvenney)
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