I've found what looks like the book, and I'm quite confused as to what he's trying to say at all. A bit further down the page he refers to Ivar's 'force of a thousand men' which is within the range of estimates for the size of the Great Heathen Army (though very much on the low side), but is absolutely tiny compared to a lot of other invasions.

Jonathan Israel calculates William III's invasion force for the 'Glorious Revolution' at slightly more than 21,000. Other estimates, that only count the Dutch regulars, still result in 14,352, or at the very least 11,212. Whatever count accepted, it's pretty obvious that the Dutch fleet of 1688, four times larger than the Spanish Armada, firing simultaneous salutes to the forts at Dover and Calais, would have dwarfed Ivars force. If the author meant 'largest invasion to date' that would make even less sense. Caesar's first invasion was made with two legions (with a theoretical strength of 10,000 men), though interestingly he refers to the 'small number' of his soldiers, and his second with five legions and 2,000 horse (theoretically 27,000 men, though the difference with previous force was probably even larger than this indicates, as they had their baggage with them this time). There were a few other invasions that were probably of comparable size (Claudius' conquest for instance), but I'm not as certain of the sources.