It is of no great bulk.
It is of no use whatever.
This book is of elegant format.
and so on.
I don't understand why don't use 'is' but 'is + preposition'? especially 'is of'
And what is difference between them?
A preposition phrase headed by of is a common way of attributing a quality to a noun, either as a modifier or as a predicate:
a man of discretion = a man who ‘possesses’ discretion, “a discreet man”.
This approach is of value. = This approach ‘possesses’ value, “it is valuable”.
This construction pushes the attributed quality to the end of the phrase or clause, the emphatic position where ‘new information’ usually appears.
The construction is particularly useful with a modified oblique—that is, of ADJ NOUN—where the alternative ADV + ADJ would be awkward:
This is a problem of challenging difficulty vs This is a ?challengingly difficult problem.
According to the Oxford Dictionary, the phrase is of means
Possess intrinsically; give rise to.
Bulk and usefulness are characteristics of something: it possesses those characteristic intrinsically, so you should say
It is of no great bulk
It is of no use whatsoever.
Similarly, something may give rise to interest, so you might say
It is of no interest to me.