Which cryptography technique does not increase the size of the plain data? I want to encrypt some data(text/voice/video) but it is essential that the size of the cipher data remains the same as the plain data.

Which techniques can I use, and how secure are these techniques as compared to the ones which increase the size of the data?
The standard answer to this question is format-preserving encryption (FPE). FPE is a class of techniques that allow you to encrypt data while preserving some of its format (which can include its length). 

In terms of security, most FPE schemes are deterministic, which means they do not achieve the standard IND-CPA notion of security. However, for high-entropy message distributions this is probably not an issue (ymmv). If your ciphertext can't be even one bit longer than the input plaintext, you don't have space to include a random IV or tweak, so randomization is out of the question unless you get tricky with your protocol.

FPE schemes usually do achieve a security notion called "strong indistinguishability from a random permutation", or SPRP. 

For an example of FPE being used in a network protocol, see this paper on network protocol obfuscation.