MFX_ERR_NOT_ENOUGH_BUFFER but MFXVideoENCODE_GetVideoParam returns same size

MFXVideoENCODE_EncodeFrameAsync is returning MFX_ERR_NOT_ENOUGH_BUFFER. But querying video parameters using following code returns the same size again as was returned by MFXVideoENCODE_Init.

    mfxStatus status;
    mfxVideoParam param;

    memset(&param, 0, sizeof(mfxVideoParam));

    status = MFXVideoENCODE_GetVideoParam(priv->session, &param);
    if (status != MFX_ERR_NONE)
    {
        return;
    }

   printf("new size = %d\n", param.mfx.BufferSizeInKB) --> This returns the same size as was returned after init of encoder.

and hence call to MFXVideoENCODE_EncodeFrameAsync is always failing after few frames when this happens.

I am using version 1.16