I'm wondering if this is a Christmas Island Frigatebird, rather than a Lesser Frigatebird.
Whereas Lesser is similar in overall length to a Black Kite, Christmas Island should appear bigger.
Based on its white head, it's a juvenile. Features mentioned by James (2004) for juvenile Christmas Island Frigatebird are:
* Axillary spurs (the white line extending from the breast across part of the underwing) long on Christmas Island and Lesser, short or absent on Greater - long
* Axillary spurs parallel-sided, angled slightly forwards - not especially parallel-sided on this bird, but definitely angled forward rather than back or positioned centrally
* Axillary spurs originate behind line of breast band - not sure
* White belly patch hexagonal (not triangular as on Lesser) - yes probably
* Alar bars (on upperwing) very prominent whitish - yes definitely
* Bill very long (cf shorter on Lesser Frigatebird) - hard to judge
Davis J. James "identification of Christmas Island, Great and Lesser Frigatebirds" BirdingASIA 1: 22-38; 2004. Author: nlinyau Time: 27/07/2015 23:44
The size is similar to Black Kite by my eyes . Author: ((華仔)) Time: 27/07/2015 23:50
I clicked this photo on last Monday (27/07/2015) in Sai Kung Pier. The Lesser Frigatebird was flying together with the Black Kites over Yeung Chau. Though far away, hope it can help comparing the sizes.