I suppose the kindest interpretation is as a reference to Verdi's greater variety of work, understood in the sense that always having (or wanting) to include Pears in every opera could be considered a constraint of sorts? ...

The extraordinary value-judgement in the quotation (presumably at Britten's expense) seems to devalue it very effectively.  I don't think BB's operatic output really stands in much need of defence? 

For all I like Britten's work, the musical devices he employs in many of the operas rarely seem truly absorbed into a coherent musical vernacular, as is certainly the case, at least to my ears, in Verdi (including in the early works). Britten picks and mixes in quite an eclectic manner, but his ideas do not often seem particularly developed (not so much in the sense of being developed through the course of a work, rather developed into something sophisticated before being employed in the work). Whereas Verdi, including in marvellous early operas such as Ernani, Attila or I Masnadierei, let alone in Rigoletto, Il Trovatore, Les vêpres siciliennes , Don Carlos and Otello, has truly absorbed a range of existing genres, techniques, styles and utterly 'made them his own'; his work is rooted in operatic tradition but rarely sounds idly derivative. Britten somehow doesn't have that same level of compositional confidence.

Damn, Mary Chambers got there first - it's at the very end of Humphrey Carpenter's biography.  The context of the previous paragraph is important:

The quote from Robert Tear is:

"There was a great, huge abyss in his soul. That's my explanation of why the music becomes thinner and thinner as time passed. He got into the valley of death and couldn't get out...   [Britten] could have been another Verdi. But he wouldn't give himself. He always stopped. He wouldn't quite go over the top."

Michael Tippett commented on that:

"That's a silly statement. You can't be like Verdi  -  that belongs to the nineteenth century. But I would have liked to change certain things for Ben, because of his immense possibilities. And there could have been some changes if he had not had Peter. But then Peter did something else for him."  

[Oh, two of you got there first  ].

Entirely so, Owain!

I can't see a fruitful purpose in comparing the output of two men who lived in entirely different times - unless the purpose of the comparison is to illustrate how the times shape the art?  (as if anyone seriously doubts this anyhow?).

Verdi lived in confident times when a belief in progress, industrialisation, and the dismantling of aristocratic privilege (Rigoletto...) into something more like a meritocracy with rosy prospects for many more than previously. Moreover, the reunification of Italy informs his work and speaks of the overthrowing of tyrannical regimes by forces of good (Macbeth, Otello). It's no coincidence that PETER GRIMES appeared in the final aftermath of WW2 (Hiroshima & Nagasaki were still months ahead), at a time when mankind first realised it was capable of annihilating itself with consummate ease.  The remainder of Britten's output was written against the background of the Cold War - the possibility of the "four minute warning" remained a threat throughout the era.  The chances Britten would want to write "like Verdi" are remote in the extreme.

(Moreover, this daft comparison excludes the fact that Verdi wrote almost exclusively for the stage...  Britten's work is far more wide-ranging and covers most other genres).

I think it is true in a sense that Britten 'wouldn't go over the top' but that only becomes a criticism if you hold the view that all composers ought to, or that 'going over the top' is a necessary condition for producing great music.

Incidentally I do think that Robert Tear's book is one of the cattiest and most snide autobiographies I have ever read which I suppose might be a commendation or a condemnation, according to taste.