This is something of a guess, but...

IgG antibodies consist of several parts. Heavy chain (~50 kDa) and light chain light chain (~25 kDa). Light Chains interact with heavy chains. Heavy chains can also interact with each other (see link).

Your bands could represent:

~150 kDa (2x Heavy+2xLight)

~75 kDa  (1x Heavy+1x Light)

~50 kDa  (1x Heavy)

The purification of the antibody may have selected only for heavy chain-containing antibody complexes (e.g. as Protein A purification does, see link).

https://www.ncbi.nlm.nih.gov/books/NBK27144/#_A325_

https://www.thermofisher.com/us/en/home/life-science/antibodies/antibodies-learning-center/antibodies-resource-library/antibody-methods/antibody-purification-methods.html

This is something of a guess, but...

IgG antibodies consist of several parts. Heavy chain (~50 kDa) and light chain light chain (~25 kDa). Light Chains interact with heavy chains. Heavy chains can also interact with each other (see link).

Your bands could represent:

~150 kDa (2x Heavy+2xLight)

~75 kDa  (1x Heavy+1x Light)

~50 kDa  (1x Heavy)

The purification of the antibody may have selected only for heavy chain-containing antibody complexes (e.g. as Protein A purification does, see link).

https://www.ncbi.nlm.nih.gov/books/NBK27144/#_A325_

https://www.thermofisher.com/us/en/home/life-science/antibodies/antibodies-learning-center/antibodies-resource-library/antibody-methods/antibody-purification-methods.html

I agree with Brian and perhaps if you re-ran the gel with about 1/10 the amount of protein you'll get a better estimate of band size and give yourself confidence that the they fall in line with what you would expect.

Brian's explanation is likely. Just to be on the safe side, however, I'd check if the antibody preparation contains carrier proteins like serum albumine.

To add to Brian Bower's answer, if you add reducing agent to your antibody, then you should only see the 25 and 50 kDa bands (though the 75 kDa looks suspiciously like albumin 66 kDa).

In my experience of antibodies on gels this pattern is quite normal: full length, light chain and heavy chain. Try heating prep at 65C for 15 minutes instead of (i'm assuming) heating to 95+ for 5 minutes and re-running.

As also suggested best to check if other additives are present 

Thanks everyone. There was nothing added in pure antibody except 2x SDS loading buffer. There was no reducing agent added to it. 

Because no reducing agents are added, Is it possible that some  of this primary antibody is degraded (disulfide bonds are broken)? 

150 kDa (2x Heavy+2xLight); 75 kDa  (1x Heavy+1x Light); 50 kDa  (1x Heavy); 25kDa (1xLight)

Because a secondary antibody in general binds to heavy chain of primary antibody, it may not detect 25kDa band (Light). 

I agree with Brian's explanation.how about add the reducing agents and see the result.